Fix: Add warning for meshes with no vertices in MeshAllocator

[Tatsuya0330]

Objective

• Fixes Spawning an Empty mesh results in "Use-after-free" error message #24874

Solution

• If a mesh contains no vertices, it now logs a WARN diagnostic message.
• Added a corresponding guard in the data copying loop to skip empty meshes, preventing the subsequent use-after-free error previously seen in the slab allocator.

Testing

• This change only adds a diagnostic warning and does not alter the rendering behavior. Since there is no existing infrastructure for asserting log output in bevy_render, I did not add a unit test for the warning itself.

Showcase

before

ERROR bevy_render::slab_allocator: Use-after-free: attempted to copy element data for an unallocated key

after

WARN bevy_render::mesh::allocator: Mesh AssetId<bevy_mesh::mesh::Mesh>{ index: 4, generation: 0} contains no vertices.

[beicause]

I think it should prefer bevy_log::warn

[Tatsuya0330]

I fixed it now.