Bug 1839204 - [mozcrash] Do not raise when signature cannot be determined r=releng-reviewers,jcristau

Sometimes the signature cannot be determined, in which case signature == None,
which usually ends up being reported "unknown top frame":
https://searchfox.org/mozilla-central/rev/c936f47f3a629ae49a4d528d3366bf29f2d4e4a7/testing/mozbase/mozcrash/mozcrash/mozcrash.py#137

Differential Revision: https://phabricator.services.mozilla.com/D181399

testing/mozbase/mozcrash/mozcrash/mozcrash.py

@@ -399,6 +399,7 @@ class CrashInfo(object):
                     signature = self._generate_signature(json_output)
 
                     # Strip parameters from signature
+                    if signature:
                         pmatch = re.search(r"(.*)\(.*\)", signature)
                         if pmatch:
                             signature = pmatch.group(1)