ci: tweak manual docker tag

Author: kelly-sovacool

.github/workflows/docker.yml

@@ -15,7 +15,7 @@ env:
   IMAGE_NAME: "scworkflow" # must be lowercase
   CONTEXT: "./"
   NAMESPACE: "nciccbr"
-  USE_INPUT_TAG: ${{ github.event_name == 'workflow_dispatch' && github.event.inputs.version_tag }}
+  USE_INPUT_TAG: ${{ github.event_name == 'workflow_dispatch' && github.event.inputs.version_tag != '' }}
 
 permissions: read-all