Add null GitHub Action.

Author: ktlim

.github/workflows/null.yaml

@@ -0,0 +1,11 @@
+name: Empty check for branch protection
+
+on:
+  - push
+  - pull_request
+
+jobs:
+  null_check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2