fix: workflow main file

GitHub changed the pull request trigger to no longer accept an empty list - I think an empty json struct should work...

Author: vsoch

.github/workflows/main.yml

@@ -2,7 +2,7 @@ name: flux-framework docs
 
 on:
   workflow_dispatch:
-  pull_request: []
+  pull_request: {}
   push:
     branches-ignore:
     - main