Fix changed-files for cases where no python files changed (#4)

## Problem
When a PR contains no changed python files, but changed-files is set to
true, ruff will run on all files, including ones not changed.

## Solution
Pass the input through to the script and short circuit to success in
cases where both the flag is set and no python files were changed.

action.yml

@@ -41,6 +41,7 @@ runs:
         INPUT_ARGS: ${{ inputs.args }}
         INPUT_SRC: ${{ inputs.src }}
         INPUT_VERSION: ${{ inputs.version }}
+        IS_CHANGED_FILES_ENABLED: ${{ inputs.changed-files }}
         CHANGED_FILES: ${{ steps.changed-files.outputs.all_changed_files }}
         pythonioencoding: utf-8
       shell: bash

action/main.py

@@ -12,6 +12,7 @@ ARGS = os.getenv("INPUT_ARGS", default="")
 SRC = os.getenv("INPUT_SRC", default="")
 VERSION = os.getenv("INPUT_VERSION", default="")
 CHANGED_FILES = os.getenv("CHANGED_FILES", "")
+IS_CHANGED_FILES_ENABLED = os.getenv("IS_CHANGED_FILES_ENABLED", default="false")
 
 version_specifier = ""
 if VERSION != "":
@@ -25,6 +26,11 @@ req = f"ruff{version_specifier}"
 # If `CHANGED_FILES` is not empty, split it into a list; otherwise, use `SRC`.
 files_to_check = shlex.split(CHANGED_FILES or SRC)
 
+# If IS_CHANGED_FILES_ENABLED is true and CHANGED_FILES was empty, there were no files to check
+# Short circuit to prevent ruff from running on all files because it got no file argument
+if IS_CHANGED_FILES_ENABLED == "true" and not CHANGED_FILES:
+    sys.exit(0)
+
 proc = run(["pipx", "run", req, *shlex.split(ARGS), *files_to_check])
 
 sys.exit(proc.returncode)