Closes https://github.com/ppy/osu/issues/26213.
Reproduction scenario: switch audio track in editor after timeline
loads.
Happens because `beatmap.Value.Track.Length` is 0 immediately after a
track switch, until BASS computes the actual track length on the audio
thread.
Yes this is a hack. No I have no better immediate ideas how to address
this otherwise.
- Checking `DrawSize != Vector2.Zero` is too specific. It could also
crash on zero-height-but-non-zero-width, or
zero-width-but-non-zero-height.
- Take the `gridCache.Validate()` call out of the zero checks, because
even if the width or height are zero, not generating anything is valid
and there is no reason to validate every frame until `gridCache` gets
invalidated again.