mapi: Return NULL function pointers for GL_EXT_debug_marker

Mesa returns a stub function pointer to glAnything for years. Android framework till API level 30 just uses function pointers returned from eglGetProcAddress without checking if the underlying extension is supported. If we return stub pointers for functions in GL_EXT_debug_marker, Android just uses our stub functions instead of its own stubs and then fail the dEQP. In the past, the issue didn't show up because mesa only has limited slots and run out of slots before Android calls eglGetProcAddress on functions inside GL_EXT_debug_marker.