Don't write buffers section if buffer is empty

[danwillm]

I have an fbx file that when procced through FBX2gLTF produced the buffers json segment as:

"buffers": [
{
"byteLength": 0
}
],

Which is not valid in gltf. Instead, omit this from being written to the file by only writing non-empty json.

[danwillm]

Hey, anything else you'd like me to resolve for this to be merged?

[fire]

I haven't had much testing of this since in Godot 4.3 we switched to ufbx. I'll merge and it'll generate builds.