Handle empty location results in live sharing

[blihio]

What changed

Handle an empty LocationResult before reading coordinates in the live location sharing callback. LocationResult.getLastLocation() returns null when its list is empty; the existing code read latitude before its later null check, causing a NullPointerException and skipping the callback's normal handling.

The callback now returns for a missing location. Non-empty results follow the existing logging and upload path.

Evidence

• XLint reported the dereference in LocationSharingUpdate.kt as Uncertain Nullable Platform Caller (possible bug, MEDIUM) in upstream commit 32bc8954ff872d0e1a05ddfae81561b6dbecef69.
• LocationResult.getLastLocation() documents and implements the empty-list return as null.
• A targeted JVM harness using the unchanged callback source and the repository's Java LocationResult/LocationCallback reproduced the NPE for an empty result. A local fixed copy completed safely for empty and non-empty results.

Closes #3889

Validation limits

The reproduction uses stubs for Android and unrelated service boundaries; it does not claim a full Android instrumentation run or a full Gradle build. No behavior outside the null guard is changed.