Fix crash for DASH manifests with empty segment timelines

DASH manifests containing an empty <SegmentTimeline/> (0 segments) in
a representation caused DashMediaPeriod.getFirstChunkStartTimeUs() to
access index 0 of an empty list, throwing an IndexOutOfBoundsException
during period preparation.

This change guards against empty segment indices before querying segment
times in DashMediaPeriod and DefaultDashChunkSource.

Issue: androidx/media#3326
PiperOrigin-RevId: 946936731
(cherry picked from commit 6c7dbd9bd89d10859ccd07a90c96cf59a1000f40)