[stf-run-ci] Update validation check for bundle URLs

An empty string passed as the bundle URL will pass the existing test
of "is defined" and "is not None" and still be invalid.

* add a check for "length != 0"
* assert requires all conditions to be met, the checks can all be listed
  separately to make the task easier to grok.

build/stf-run-ci/tasks/setup_stf_from_bundles.yml

@@ -90,10 +90,14 @@
 - name: "Ensure that the bundle paths are set."
   ansible.builtin.assert:
     that:
-      - '__smart_gateway_bundle_image_path is defined and __smart_gateway_bundle_image_path != None'
-      - '__service_telemetry_bundle_image_path is defined and __service_telemetry_bundle_image_path != None'
+      - '__smart_gateway_bundle_image_path is defined'
+      - '__smart_gateway_bundle_image_path != None'
+      - '__smart_gateway_bundle_image_path | length != 0'
+      - '__service_telemetry_bundle_image_path is defined'
+      - '__service_telemetry_bundle_image_path != None'
+      - '__service_telemetry_bindle_image_path | length != 0'
     fail_msg: "Bundle path(s) not set. __smart_gateway_bundle_image_path is '{{ __smart_gateway_bundle_image_path }}' and __service_telemetry_bundle_image_path is '{{ __service_telemetry_bundle_image_path }}'. Both values need to be set."
-    success_msg: "Bundle paths are defined and not None"
+    success_msg: "Bundle paths are defined, are not None and have a non-zero-length"
 
 - name: Deploy SGO via OLM bundle
   ansible.builtin.shell: