Check against empty strings as errors in bundle code

The result.errors from `GetChangesMapArgs` has changed in Juju to
return something like `['', '', '', '', '', '', '', '', '']` instead
of `[]` in a non-error return.

Author: cderici

juju/bundle.py

@@ -298,7 +298,7 @@ async def fetch_plan(self, bundle_url, origin, overlays=[]):
             bundleurl=entity_id,
             yaml=yaml_data)
 
-        if self.plan.errors:
+        if self.plan.errors and any(self.plan.errors):
             raise JujuError(self.plan.errors)
 
     async def _download_bundle(self, charm_url, origin):