Fix getting base_ref for non-PRs

Part of #68.

Author: krzema12

.github/workflows/test.main.kts

@@ -81,7 +81,7 @@ workflow(
         run(
             name = "Check for actions",
         ) {
-            validateTypings(github.sha, github.base_ref)
+            validateTypings(github.sha, github.base_ref?.ifEmpty { null })
         }
     }