glTF2s with blend weights being zero is being rejected

[fire]

Godot version:
3.2 stable from Steam

OS/device including version:
Win10

Issue description:

I had a model with blend shapes defined but with 0 blend weights. It is not required to be in the model format and should not be rejected with a parse error.

https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#mesh

ERR_FAIL_COND_V(mesh->blend_weights.size() == 0, ERR_PARSE_ERROR);

godot/editor/import/editor_scene_importer_gltf.cpp

Line 2393 in 00949f0

ERR_FAIL_COND_V(mesh->blend_weights.size() == 0, ERR_PARSE_ERROR);

The fix is to be determined.

Steps to reproduce:

Import a gltf2 with the blend weights array deleted and it will fail to import.

Minimal reproduction project:

38751.zip

From https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/AnimatedMorphCube but with https://github.com/KhronosGroup/glTF-Sample-Models/blob/5aec133dbaf543f9bcb6cb79de9966bf9530c2fe/2.0/AnimatedMorphCube/glTF/AnimatedMorphCube.gltf#L235-L238 deleted.

[slapin]

aw these assert reverts :( totally flip my brain...

[slapin]

Anyway are you sure these checks are necessary? meshes work fine without weights.