[medium] fix: [mobilebackup] is_compatible() raises TypeError when the case model is unknown

[elhoim]

BLUF

• Priority: medium. MobileBackupParser.is_compatible() raises TypeError: argument of type 'NoneType' is not iterable whenever the case metadata carries no model.
• case_model is documented as optional. BaseInterface.case_model is typed str | None and returns self.case.get("model"), so None is a legitimate, expected value — but the device check does "AppleTV" not in self.case_model with no guard.
• The blast radius is the whole run, not this parser. BaseInterface._execute_and_write() calls is_compatible() outside its own try/except crash guard, so the TypeError is not caught, escapes Sysdiagnose.parse(), and aborts the entire invocation — every remaining parser and every remaining case is skipped.
• Fix: coalesce to "" before the substring test. A case with an unknown model is treated as compatible, which is the safe default for a forensic tool — better to run the parser and produce nothing than to skip it silently.
• Scope: 3 changed lines in one parser, plus a regression test. No base-class change.

Reproducing

Any case whose cases.json entry has no model key — which happens whenever get_case_metadata() could not determine the device model from the archive:

case = {"case_id": "no-model", "ios_version": "16.0"}
MobileBackupParser(config, case=case).is_compatible()
# TypeError: argument of type 'NoneType' is not iterable

The fix

# case_model is None when the case metadata carries no model; treat that as compatible
case_model = self.case_model or ""
device_compatibility = "AppleTV" not in case_model and "Watch" not in case_model

This parser is not compatible with Apple TV or Apple Watch, and that exclusion is preserved exactly.

Test

test_is_compatible_without_case_model builds a case dict with no model, asserts case_model is None, and asserts is_compatible() returns a bool instead of raising. Verified to fail on main with the exact TypeError above and pass with this change.

Note

Sibling parsers (mcstate_shared_profile, itunesstore, networkextension) carry the identical unguarded pattern. They are submitted as separate PRs so each can be reviewed and reverted independently.