Support for accessors with no `bufferView`

[atteneder]

Describe the bug

The glTF specification says the following about accessors' bufferView attribute:

When undefined, the accessor MUST be initialized with zeros;

Source: https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#_accessor_bufferview

Currently glTFs with such accessors cause issues like AssertionExceptions.

Expected behavior

glTF files with such accessors should load without errors.

• glTFast version 6.8.0