Stop the labeler removing labels

The sync-labels option was expected to accept "true" or "false" with "false"
being the default.  However, these are currently being converted to boolean
using the Ecmascript `!` operator, which converts both of those strings to
True.  Pass the empty string instead to get the value False.

https://github.com/actions/labeler/issues/104

.github/workflows/labeler.yml

@@ -9,3 +9,4 @@ jobs:
     - uses: actions/labeler@main
       with:
         repo-token: "${{ secrets.GITHUB_TOKEN }}"
+        sync-labels: ""