pr-check: Fix some minor bugs

.github/workflows/pr-check.yml

@@ -88,12 +88,12 @@ jobs:
             PR_TITLE=$(echo "$json" | jq -r .title)
             NORMALIZED_PR_BODY=$(printf "%s\n" "$PR_BODY" | tr -d '\r')
 
-            unset "$PR_TITLE"
+            unset PR_TITLE
             export PR_TITLE="$PR_TITLE"
 
             EXIT_CODE=$(curl -sSL "$SCRIPT_URL" | python3 | grep 'EXIT_CODE=' | cut -d= -f2)
             echo "EXIT_CODE is $EXIT_CODE"
-            unset "$PR_TITLE"
+            unset PR_TITLE
 
             PR_COMMENTS="$(gh pr view "$PR_NUM" --json comments -q '.comments[] | select(.author.login == "flathubbot" or .author.login == "github-actions") | .body')"
 
@@ -129,6 +129,7 @@ jobs:
             }
 
             PR_THREAD_COUNT=$(get_unresolved_review_threads flathub flathub $PR_NUM)
+            PR_THREAD_COUNT=${PR_THREAD_COUNT:-0}
 
             PR_FILES=$(gh pr view "$PR_NUM" --json files --jq '.files[].path')
 
@@ -185,7 +186,7 @@ jobs:
               fi
             }
 
-            if [ "$CONTAINS_MASTER" = "true" ]; then
+            if [ "${CONTAINS_MASTER:-false}" = "true" ]; then
               echo "PR contains commits from master branch"
               REVIEW_COMMENT_LINES+=("- PR does not contain commits from the [master branch](https://github.com/flathub/flathub/commits/master/)")
               BLOCKED=1