osbuild: return if output-directory not specified

If the user does not specify an output directory or checkpoints
to osbuild, exit successfully without building.

Previously, if a user did not include an output directory or
checkpoints, it would build the manifest and throw out the result.
Returning early will be clearer to the user and avoid wasting work.

osbuild/main_cli.py

@@ -141,6 +141,10 @@ def osbuild_cli():
         sys.stdout.write("\n")
         return 0
 
+    if not args.output_directory and not args.checkpoint:
+        print("No output directory or checkpoints specified, exited without building.")
+        return 0
+
     try:
         r = pipeline.run(
             args.store,