allow transition functions to return nothing

[Rich-Harris]

Allows a transition function to return nothing, indicating no transition should take place.

There's a better way to do this, which is not running the transition at all (rather than running and immediately canceling), but it involves some fiddling to ensure outros still happen at the correct time, so I'm just going to leave this as a note to self

[codecov-io]

Codecov Report

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The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #2246   +/-   ##
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  Coverage          ?   91.83%           
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  Files             ?        1           
  Lines             ?       49           
  Branches          ?        0           
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  Hits              ?       45           
  Misses            ?        4           
  Partials          ?        0

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[Conduitry]

Does it make sense to just merge this in its current form and open up a separate issue for optimizing this?

[Rich-Harris]

yeah, fair enough. brought it up to date with master, but there's some TypeScript bullshit to deal with

[Conduitry]

What do you think about a similar change for #3300 where we have a null_animation object with a constantly 0 easing function?

[Rich-Harris]

That seems reasonable, yeah. I couldn't quite tell if { duration: 0 } would work or if it needs to override the easing function, but either way, something along those lines.