mrhlpr: make "unknown" mrhlpr.json section optional

There's no need for it, and breaks when trying to merge pmaports!5840 (comment 457021)

mrhlpr/mr.py

@@ -410,7 +410,7 @@ def commits_follow_format(commits: list[str]) -> tuple[Optional[bool], list[Any]
         regexes_pass.append(re.compile(regex))
 
     regexes_unknown = []
-    for regex in definitions["subject_format"]["unknown"]:
+    for regex in definitions["subject_format"].get("unknown", []):
         regexes_unknown.append(re.compile(regex))
 
     result: Optional[bool] = True