replace NULL with 0

sokol_gfx.h

@@ -7044,7 +7044,7 @@ _SOKOL_PRIVATE void _sg_gl_reset_state_cache(void) {
 
 _SOKOL_PRIVATE void _sg_gl_setup_backend(const sg_desc* desc) {
     _SOKOL_UNUSED(desc);
-    SOKOL_ASSERT(desc->context.gl.default_framebuffer_cb == NULL || desc->context.gl.default_framebuffer_userdata_cb == NULL);
+    SOKOL_ASSERT(desc->context.gl.default_framebuffer_cb == 0 || desc->context.gl.default_framebuffer_userdata_cb == 0);
     
     // assumes that _sg.gl is already zero-initialized
     _sg.gl.valid = true;