fix: address code review feedback

Resolve review comments from @yolabingo:

1. Use JSON empty array [] instead of empty string for files output
   when no YAML files are changed. This ensures consistent JSON format
   for downstream processing.

2. For push events, use github.event.before and github.event.after
   instead of default branch comparison. This provides the correct
   commit range for push events as suggested.

Author: dcolina

.github/workflows/deployment-guard.yml

@@ -187,15 +187,16 @@ jobs:
             BASE_SHA="${{ github.event.pull_request.base.sha }}"
             HEAD_SHA="${{ github.event.pull_request.head.sha }}"
           else
-            BASE_SHA="origin/${{ github.event.repository.default_branch }}"
-            HEAD_SHA="HEAD"
+            # For push events, use event.before and event.after for correct commit range
+            BASE_SHA="${{ github.event.before }}"
+            HEAD_SHA="${{ github.event.after }}"
           fi
 
           CHANGED_FILES=$(git diff --name-only "$BASE_SHA" "$HEAD_SHA" | grep -E '\.ya?ml$' || true)
 
           if [ -z "$CHANGED_FILES" ]; then
             echo "No YAML files changed"
-            echo "files=" >> "$GITHUB_OUTPUT"
+            echo "files=[]" >> "$GITHUB_OUTPUT"
           else
             echo "Changed files:"
             echo "$CHANGED_FILES"