Remove quotes around $DRAFT argument

gh will complain if there is an empty `""`, which is the case if
`$DRAFT` is unset.

.github/workflows/post-release-mergeback.yml

@@ -124,4 +124,4 @@ jobs:
             --base "$BASE_BRANCH" \
             --title "$PR_TITLE" \
             --body "$PR_BODY" \
-            "$DRAFT"
+            ${DRAFT:+"$DRAFT"} # no quotes around $DRAFT. gh will error out if there is an empty ""