Crash introduced by {rB0cb5eae}.
When switching to between drawing modes the region.draw_buffer could be
uninitialized when the gizmo depth test is performed. When the mouse is
placed on top of a gizmo part that could be highlighted would crash.
This fix adds a early exit when depth testing is requested, but there
isn't a draw_buffer. Not sure this is an root cause fix.
Reported by multiple animators in Blender Studio.