Don't error if there are no files to shellcheck

pre-commit.shellcheck

@@ -33,7 +33,9 @@ function main {
 		files[${files[@]}+1]="$file"
 	done < <(get_shell_files)
 
+	if [[ "${#files[@]}" -ne 0 ]]; then
 		shellcheck "${files[@]}"
+	fi
 }
 
 main