Fix invalid sparse buffer update pipeline queued on WebGL2

[beicause]

Objective

Fixes #24944

#23434 queues the sparse buffer update pipeline with an invalid shader handle which causes the pipeline creation never to finish.

bevy/crates/bevy_render/src/render_resource/sparse_buffer_vec.rs

Line 296 in 25368b7

shader: self.shader.clone().unwrap_or_default(),

Solution

Let specialize take a shader handle as key and don't specialize the pipeline if shader handle is None.
I also converted FromWorld to RenderStartup.

Testing

Tested loading_screen example on web