fix(readme): Add a test for empty file list in git hook.

As it was, the proposed git hook will fail if no Julia file has been
edited in the commit (for example if we only edited Project.toml).

README.md

@@ -213,7 +213,10 @@ exec 1>&2
 
 # Run Runic on added and modified files
 mapfile -t files < <(git diff-index --name-only --diff-filter=AM master | grep '\.jl$')
+
+if [ ! -z "${files}" ]; then
     julia --project=@runic -m Runic --check --diff "${files[@]}"
+fi
 ```
 
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