Empty track segment flips GPX.has_times() to True, blocking fill_time_data_with_regular_intervals

[Str0k]

An empty track segment makes GPXTrack.has_times() return True even when every point in the track lacks a timestamp. Consequently, fill_time_data_with_regular_intervals(force=False) rejects a file that has no time data.

Restore AND aggregation across segments. An empty segment then remains neutral, as documented by GPXTrackSegment.has_times(). The change also restores the behavior before commit 81cb1eb. Three new unittest cases cover time availability, filling missing times, and a valid timed track; they are included in the repository's python -m unittest test entry point.

Validation against base df845044cc87ba6401e494a3a177ed007ff0f6eb:

Check	Base	Patch
New regression module	2 failed, 1 passed	3 passed
Native python -m unittest test	149 passed	152 passed
gpxinfo on repository fixtures	Passed	Passed
Strict Mypy	19 existing diagnostics	Same 19; no additions

The regression and complete suites were reproduced in independent clean checkouts. The separate pytest adapter also passed after the patch; the native unittest counts above avoid its duplicate collection of imported tests.

AI assistance: implementation and independent code review used Hermes with self-hosted GLM 5.3. Automated validation was reproduced separately. This does not represent a human review.