bpo-40548: github actions: pass the changes check on no source changes (GH-20097)

Signed-off-by: Filipe Laíns <[email protected]>

Author: FFY00

.github/workflows/build.yml

@@ -30,7 +30,7 @@ jobs:
             echo '::set-output name=run_tests::true'
           else
             git fetch origin $GITHUB_BASE_REF --depth=1
-            git diff --name-only origin/$GITHUB_BASE_REF... | grep -qvE '(\.rst$|^Doc|^Misc)' && echo '::set-output name=run_tests::true'
+            git diff --name-only origin/$GITHUB_BASE_REF... | grep -qvE '(\.rst$|^Doc|^Misc)' && echo '::set-output name=run_tests::true' || true
           fi
   build_win32:
     name: 'Windows (x86)'