[nrf fromlist] scripts: ci: test_plan: use find_modules only when commits are provided

Current implementation will not work if comits were not provided.
ie. use case with list of changed files will fail as args.commits is None.

Upstream PR: zephyrproject-rtos/zephyr#75927

Signed-off-by: Piotr Kosycarz <[email protected]>

Author: nordic-piks

scripts/ci/test_plan.py

@@ -149,7 +149,7 @@ def get_plan(self, options, integration=False, use_testsuite_root=True):
             os.remove(fname)
 
     def find_modules(self):
-        if 'west.yml' in self.modified_files:
+        if 'west.yml' in self.modified_files and args.commits is not None:
             print(f"Manifest file 'west.yml' changed")
             print("=========")
             old_manifest_content = repo_to_scan.git.show(f"{args.commits[:-2]}:west.yml")