Tombstone: fall back to file timestamp on empty pb timestamp

[besendorf]

• Allow empty protobuf timestamps to fall back to the file timestamp
• Update _parse_timestamp_string to accept a fallback
• Pass file_timestamp to its callers
• Add tests for empty and whitespace tombstone timestamps
• Adjust imports in bugreport module to include DumpsysADBState Tombstone: use file timestamp on empty pb
• skip 0 bytes tombstones and show debug warning

[github-actions]

Tests	Skipped	Failures	Errors	Time
113	1 💤	0 ❌	0 🔥	9.787s ⏱️

[bitmeta69]

Thanks for working on this. I independently confirmed the zero-byte case against the released MVT v2026.7.29. A consented Android 13 bugreport contained 25 zero-byte FS/data/tombstones/tombstone_*.pb placeholder entries, and the Tombstones module emitted 25 String does not contain a date errors.

The failure also reproduces without device data:

from datetime import datetime, timezone
from mvt.android.modules.bugreport.tombstones import Tombstones

Tombstones().parse_protobuf(
    "tombstone_00.pb",
    datetime(2026, 1, 1, tzinfo=timezone.utc),
    b"",
)

This raises ParserError: String does not contain a date:. The early zero-length check in this PR appears to address the exact observed case. I can validate a rebased revision against the preserved acquisition if that would be useful.