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

@@ -399,9 +399,10 @@ class CrashInfo(object):
                     signature = self._generate_signature(json_output)
 
                     # Strip parameters from signature
-                    pmatch = re.search(r"(.*)\(.*\)", signature)
-                    if pmatch:
-                        signature = pmatch.group(1)
+                    if signature:
+                        pmatch = re.search(r"(.*)\(.*\)", signature)
+                        if pmatch:
+                            signature = pmatch.group(1)
 
         else:
             if not self.stackwalk_binary: