ci: Fix another branch name issue

.github/scripts/apidoc.sh

@@ -40,7 +40,7 @@ cd build/apidoc
 # This will re-use already existing branches (updated PR).
 if [ "$PR_NUMBER" != "" ]; then
     BRANCH="pr-$PR_NUMBER"
-elif [ "$GITHUB_HEAD_REF" != master ]; then
+elif ["$GITHUB_HEAD_REF" != "" ] && [ "$GITHUB_HEAD_REF" != master ]; then
     # Use merge-base of master in branch name, to keep different branches with
     # the same name apart.
     # shellcheck disable=SC2015