Clean up Shader Module getUniforms semantics

[ibgreen]

PR #1907 is a change to the shader module getUniform semantics - an empty call no longer generates uniforms. I think it is a good change (though the system is still a bit messy).

Also this default value handling was previously handled by the getUniforms function returned by the shader assembler.

Would be good to

• see if we can remove the old shader assembler getUniforms code so we don't have divergent semantics in different places.
• make sure the documentation reflects the latest semantics
• if needed, add an upgrade guide note.