Add a test for empty file list in git hook suggestion (#26)

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 [ ${#files[@]} -gt 0 ]; then
     julia --project=@runic -m Runic --check --diff "${files[@]}"
+fi
 ```
 
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