Fix error with empty style definitions json file

An artifact in our own codebase could sometimes cause empty style
definitions json files. This will fail compilation of ex_css_modules
because decoding an empty string is not working.

To capture this edge case, let's check if the file is empty before
trying to decode OR generate the json file through a task.

Author: EasterPeanut

README.md

@@ -7,7 +7,11 @@
 
 ExCSSModules defines two ways to read the stylesheet: embedded and read.
 
-If you set the `embed_stylesheet` option to the `use` macro the stylesheet definitions JSON have to be compiled before the application is compiled. This flag is used for production to optimize read times. Additionally, you can add a task to generate missing stylesheet definitions JSON files when compiling ExCSSModules, to prevent compilation errors when one of the stylesheet definitions JSON might be missing:
+If you set the `embed_stylesheet` option to the `use` macro the stylesheet definitions JSON have to be compiled before the application is compiled. This flag is used for production to optimize read times. 
+
+If you don't set the flag or set it to false, the stylesheet definition JSON files are read live from the server which creates a lot of IO for each request.
+
+Additionally, you can pass a task to generate missing stylesheet definitions JSON files when compiling ExCSSModules, to prevent compilation errors when one of the stylesheet definitions JSON might be missing:
 
 ```ex
 defmodule Mix.Tasks.GenerateMissingJson do
@@ -28,7 +32,18 @@ defmodule Mix.Tasks.GenerateMissingJson do
 end
 ```
 
-If you don't set the flag or set it to false, the stylesheet definition JSON files are read live from the server which creates a lot of IO for each request.
+This task can be passed through the option `build_stylesheet_definitions_json_task`: 
+
+```ex
+use ExCSSModules.View,
+  namespace: MyApplicationWeb,
+  embed_stylesheet: ApplicationSettings.built_for?(:prod),
+  build_stylesheet_definitions_json_task: unquote(Mix.Tasks.BuildScopedJson),
+  stylesheet:
+    __ENV__.file
+    |> Path.dirname()
+    |> Path.join("./style.css")
+```
 
 ## Installation
 Install from [Hex.pm](https://hex.pm/packages/ex_css_modules):

lib/ex_css_modules/ex_css_modules.ex

@@ -38,8 +38,8 @@ defmodule ExCSSModules do
 
   defp read_stylesheet(filename, build_json_task) do
     cond do
-      File.exists?(filename <> ".json") ->
-        (filename <> ".json")
+      File.exists?("#{filename}.json") && !empty_file?("#{filename}.json") ->
+        "#{filename}.json"
         |> File.read!()
         |> json_library().decode!()
 
@@ -55,6 +55,8 @@ defmodule ExCSSModules do
     end
   end
 
+  defp empty_file?(filename), do: File.read!(filename) == ""
+
   @doc """
   Reads the class definitions from the definition map and maps them to a class
   attribute. The `keys` argument is used to retrieve the class name or multiple