c2a: Do not assume test_mapping.json exists

update_crate_tests would previously assume test_mapping.json existed
when trying to generate a TEST_MAPPING update. This made updating
mappings for the majority of existing crates fail.

Bug: 233924440
Test: Regenerate all Rust TEST_MAPPINGs
Change-Id: I4d9b906f4db233ee0223a3035c1b63bf1ca681f4
diff --git a/scripts/update_crate_tests.py b/scripts/update_crate_tests.py
index 1795de2..7bd9f05 100755
--- a/scripts/update_crate_tests.py
+++ b/scripts/update_crate_tests.py
@@ -352,7 +352,10 @@
                 subprocess.check_output(['repo', 'start',
                                          'tmp_auto_test_mapping', '.'])
                 subprocess.check_output(['git', 'add', 'TEST_MAPPING'])
-                subprocess.check_output(['git', 'add', 'test_mapping_config.json'])
+                # test_mapping_config.json is not always present
+                subprocess.call(['git', 'add', 'test_mapping_config.json'],
+                                stderr=subprocess.DEVNULL,
+                                stdout=subprocess.DEVNULL)
                 subprocess.check_output(['git', 'commit', '-m',
                                          'Update TEST_MAPPING\n\nTest: None'])
             if args.push_change and (changed or untracked):