safe-merge-queue should gracefully handle network failures with github

https://bugs.webkit.org/show_bug.cgi?id=263249
rdar://117172642

Reviewed by Jonathan Bedard.

Adds error handling for when query returns None.

* Tools/CISupport/ews-build/steps.py:
(GitHubMixin.query_graph_ql):
(RetrievePRDataFromLabel.run):

Canonical link: https://commits.webkit.org/269534@main

Tools/CISupport/ews-build/steps.py

@@ -232,6 +232,8 @@ def query_graph_ql(self, payload):
         if response and response.status_code // 100 != 2:
             yield self._addToLog('stdio', f'Accessed {graphql_url} with unexpected status code {response.status_code}.\n')
             defer.returnValue(False if response.status_code // 100 == 4 else None)
+        elif not response:
+            defer.returnValue(False)
         else:
             data = json.loads(response.text)
             defer.returnValue(data)
@@ -2369,6 +2371,7 @@ def run(self):
             yield self._addToLog('stdio', f'Ending process as there are no PRs in {self.label}.\n')
             return defer.returnValue(SUCCESS)
         if not num_prs:
+            yield self._addToLog('stdio', f'Failed to retrieve number of PRs in {self.label}.\n')
             return defer.returnValue(FAILURE)
 
         self.setProperty('passed_status_check', [])