Ignore text keys for zero-length animations (Fixes #1876)

This fixes an issue where the Riekling's attack animation would trigger *two* hits on each swing. It has the "min hit" and "hit" keys at the same time, so the MinHitToHit segment a zero-length animation. This caused problems as the "hit" text key will be encountered twice; once when playing the "max attack to min hit" segment and once when playing the "min hit to hit" segment.