Assorted fixes for more robust handling of asset corner cases

[mosra]

Motivation and Context

• Enables shader texture transform only if it's actually textured. If it's not (for example if a texture failed to load or if the material data are strange in some way), then the shader construction would assert with

  Shaders::PhongGL: texture transformation enabled but the shader is not textured
  

  This change doesn't fix any root causes of a texture missing or failing to load, it only makes it not abort anymore. Also removed a now-irrelevant TODO and a branch in flat material data setup -- that check should have been done in the shader setup code in the first place anyway, which is how it's done now. The TODO was about a different thing, reverted that back.

• Fixed a crash if the BasisImporter plugin wouldn't be present for whatever reason. (I disabled it in order to be able to test the above assert, and discovered this crash.)

• And fixed a case where files without a default scene would be treated as having no scene at all. This is the case for example with glTF files produced by gltfpack.

How Has This Been Tested

With the BasisImporter plugin disabled, loading gltfpack-produced files shared by @mukulkhanna no longer dies with the above shader assertion.

Originally the assertion was a consequence of this error:

Trade::AnyImageImporter::openData(): cannot load the KtxImporter plugin

... which as far as I can tell isn't reproducible on current main anymore. It was fixed by mosra/magnum@569ae5a (which delegates to BasisImporter if KtxImporter isn't found), which was merged into Habitat with #1853.

Types of changes

• Bug fix (non-breaking change which fixes an issue)