Add null GitHub Actions workflow

This will be useful for setting up branch protections to ensure the
branch is up-to-date. However, hopefully we can add more useful linting
in the future.

.github/workflows/ci.yaml

@@ -0,0 +1,12 @@
+name: Empty check for branch protection
+
+on:
+  - push
+  - pull_request
+
+jobs:
+
+  null_check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2