fix: cpu pegging when shpool attach stdin is null

[ethanpailes]

This patch fixes an issue where we would peg the CPU if the stdin to shpool attach was null.

We handle this by immediately exiting the attach proc if stdin is null.

Fixes #243

Replaces #246

[maxhbooth]

looks like there are some cranky errors:

error: length comparison to zero
    --> shpool/tests/attach.rs:1592:12
     |
1592 |         if list_blob["sessions"].as_array().unwrap().len() == 0 {
     |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: using `is_empty` is clearer and more explicit: `list_blob["sessions"].as_array().unwrap().is_empty()`

[ethanpailes]

Yeah, true. Should be fixed now.