fix: Ensure empty sources have one "line"

[Muscraft]

rustc will occasionally give us a Snippet with an empty source and an Annotation with the span 0..0, which causes annotate-snippets to panic. The root cause of the panic is that a few places in SourceMap expect at least one LineInfo, when none are created for an empty source. To fix this, I made it so when we are given an empty source, we add a single "empty" LineInfo, with the appropriate line_index.

---

When looking into how to fix this, I noticed that rustc doesn't show the annotation even though there is one that could be displayed. Because of this, I felt it was best to diverge from how rustc would render this and show the annotation (and the correct line).

Original Test

#[test]
fn mismatched_types1() {
    // tests/ui/include-macros/mismatched-types.rs

    let file_txt_source = r#""#;

    let rust_source = r#"fn main() {
    let b: &[u8] = include_str!("file.txt");    //~ ERROR mismatched types
    let s: &str = include_bytes!("file.txt");   //~ ERROR mismatched types
}"#;

    let input = Level::ERROR.header("mismatched types").id("E0308").group(
        Group::new()
            .element(
                Snippet::source(file_txt_source)
                    .fold(true)
                    .line_start(3)
                    .origin("$DIR/file.txt")
                    .annotation(
                        AnnotationKind::Primary
                            .span(0..0)
                            .label("expected `&[u8]`, found `&str`"),
                    ),
            )
            .element(
                Snippet::source(rust_source)
                    .origin("$DIR/mismatched-types.rs")
                    .fold(true)
                    .annotation(
                        AnnotationKind::Context
                            .span(23..28)
                            .label("expected due to this"),
                    )
                    .annotation(
                        AnnotationKind::Context
                            .span(31..55)
                            .label("in this macro invocation"),
                    ),
            )
            .element(
                Level::NOTE.title("expected reference `&[u8]`\n   found reference `&'static str`"),
            ),
    );

    let expected = str![[r#"
error[E0308]: mismatched types
  --> $DIR/file.txt:0:1
   |
   |
  ::: $DIR/mismatched-types.rs:2:12
   |
LL |     let b: &[u8] = include_str!("file.txt");    //~ ERROR mismatched types
   |            -----   ------------------------ in this macro invocation
   |            |
   |            expected due to this
   |
   = note: expected reference `&[u8]`
              found reference `&'static str`
"#]];
    let renderer = Renderer::plain().anonymized_line_numbers(true);
    assert_data_eq!(renderer.render(input), expected);
}

Proposed

#[test]
fn mismatched_types1() {
    // tests/ui/include-macros/mismatched-types.rs

    let file_txt_source = r#""#;

    let rust_source = r#"fn main() {
    let b: &[u8] = include_str!("file.txt");    //~ ERROR mismatched types
    let s: &str = include_bytes!("file.txt");   //~ ERROR mismatched types
}"#;

    let input = Level::ERROR.header("mismatched types").id("E0308").group(
        Group::new()
            .element(
                Snippet::source(file_txt_source)
                    .fold(true)
                    .line_start(3)
                    .origin("$DIR/file.txt")
                    .annotation(
                        AnnotationKind::Primary
                            .span(0..0)
                            .label("expected `&[u8]`, found `&str`"),
                    ),
            )
            .element(
                Snippet::source(rust_source)
                    .origin("$DIR/mismatched-types.rs")
                    .fold(true)
                    .annotation(
                        AnnotationKind::Context
                            .span(23..28)
                            .label("expected due to this"),
                    )
                    .annotation(
                        AnnotationKind::Context
                            .span(31..55)
                            .label("in this macro invocation"),
                    ),
            )
            .element(
                Level::NOTE.title("expected reference `&[u8]`\n   found reference `&'static str`"),
            ),
    );

    let expected = str![[r#"
error[E0308]: mismatched types
  --> $DIR/file.txt:3:1
   |
LL |
   | ^ expected `&[u8]`, found `&str`
   |
  ::: $DIR/mismatched-types.rs:2:12
   |
LL |     let b: &[u8] = include_str!("file.txt");    //~ ERROR mismatched types
   |            -----   ------------------------ in this macro invocation
   |            |
   |            expected due to this
   |
   = note: expected reference `&[u8]`
              found reference `&'static str`
"#]];
    let renderer = Renderer::plain().anonymized_line_numbers(true);
    assert_data_eq!(renderer.render(input), expected);
}

Note: I will try to backport this change to rustc so that this is only a temporary divergence in output.