GLTFExporter: Fix undefined node name bug in mergeMorphTargetTracks()

[takahirox]

Related issue: Resolves #20792
Related PR: #19042

Description

This PR resolves an issue that ""GLTFExporter: parse - individual morph targets" unit test never ends". Refer to #20792 for the detail about the problem and the root issue.

/cc @donmccurdy

[mrdoob]

Where does sourceTrackBinding.nodeName come from?
We usually initialise foo.name as '' 🤔

[takahirox]

PropertyBinding.parseTrackName()

https://github.com/mrdoob/three.js/blob/dev/src/animation/PropertyBinding.js#L141

sourceTrackBinding is the result of this regex https://github.com/mrdoob/three.js/blob/dev/src/animation/PropertyBinding.js#L1-L33 The regex allows some of properties can be unmatch (undefined).

We might be able to force to empty string rather than undefined in parseTrackName() side? I'm not sure how the change affects the existing modules tho.

[Mugen87]

We might be able to force to empty string rather than undefined in parseTrackName() side?

I vote for this approach.

Although I think the PR can still be merged and the refactoring done with a separate one later.

[mrdoob]

Sounds good 👍

[mrdoob]

Thanks!