Fix NULL RTV and clearing with unbound draw buffers
Fixes https://gitlab.freedesktop.org/kusma/mesa/-/issues/78
Also fixes:
spec@arb_framebuffer_object@fbo-drawbuffers-none glclear
spec@arb_framebuffer_object@fbo-drawbuffers-none glclearbuffer
However, due to a bug in the validation layer, there is still a warning displayed.