…ck distance is zero

RFC. This closes ppy#31186.

To explain why: The issue occurs on
https://osu.ppy.sh/beatmapsets/594828#osu/1258033, specifically on the
slider at time 128604. The failure site is

	https://github.com/ppy/osu/blob/fa0d2f4af22fb9319e2a8773bf635368d86360be/osu.Game.Rulesets.Osu/Edit/Blueprints/Sliders/SliderCircleOverlay.cs#L65-L66

wherein `LastRepeat` is `null`, even though the slider's `RepeatCount`
is 1 and thus `SpanCount` is 2.

In this case, `SliderEventGenerator` is given a non-zero `tickDistance`
but a zero `length`. The former is clamped to the latter:

	https://github.com/ppy/osu/blob/fa0d2f4af22fb9319e2a8773bf635368d86360be/osu.Game/Rulesets/Objects/SliderEventGenerator.cs#L34

Because of this, a whole block of code pertaining to tick generation
gets turned off, because of zero tick spacing - however, that block also
includes within it *repeat* generation, for seemingly very little reason
whatsoever:

	https://github.com/ppy/osu/blob/fa0d2f4af22fb9319e2a8773bf635368d86360be/osu.Game/Rulesets/Objects/SliderEventGenerator.cs#L47-L77

While a zero tick distance would indeed cause `generateTicks()` to loop
forever, it should have absolutely no effect on repeats.

While this *is* ultimately an aspire-tier bug caused by people pushing
things to limits, I do believe that in this case a fix is warranted
because of how hard the current behaviour violates invariants. I do not
like the possibility of having a slider with multiple spans and no
repeats.