Do not try updating docs if no key is set

This will avoid this randomly failing in forks, unless we want to run it

.github/workflows/update-master-docs.yml

@@ -16,6 +16,7 @@ on:
 
 jobs:
   build:
+    if: ${{ secrets.SSH_KEY_SECRET }} != ${{ '' }}
     runs-on: ubuntu-18.04
     steps:
       - name: Checkout rgbds@master