Fix AnimationNode's branching by custom timeline usage

[TokageItLab]

What problem(s) does this PR solve?

• Closes AnimationTree: The Animations play at high speed #119979
• Follow up Make AnimationNodeAnimation's custom timeline processing logic compatible with AnimationMixer #119871

Since the default value for stretch_time_scale is true, the code would check it even when use_custom_timeline was disabled, causing it to use an uninitialized timeline_length in the speed calculation. Fixed the check. cc @chocola-mint

Since the default value for timeline_length is 1.0 seconds, issues arise with animations that do not exactly 1.0 second duration. I didn’t notice this in #119871 test because the test used animations with 1.0 seconds duration. If we’re creating tests, we should include animations that do not 1.0 seconds duration.

Additional information

[Repiteo]

Thanks!