new_render_command_encoder assumes the return value can never be NULL

[madsmtm]

Found while working on #241.

The argument-buffer and the bindless examples currently pass an empty RenderPassDescriptor to CommandBuffer::new_render_command_encoder, which causes the function to return NULL (instead of an actual render command encoder object).

This in turn means that all the code following it basically does nothing (as it sends messages to a NULL value, which Objective-C just ignores).

To very clearly see how this is unsound and confusing, try putting the following statement after new_render_command_encoder:

let encoder = Some(encoder).unwrap();