shaped-texture: Use actual texture if tower returned NULL

The texture tower can return no texture e.g. if the calculated level is
negative. This was handled before, but regressed with
e1370ee2. This fixes a potential crash
observed occasionally when starting Firefox nightly using the Wayland
backend in overview mode.
2 jobs for wip/handle-texture-tower-null in 5 minutes and 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #180351
build-mutter

00:01:20

 
  Test
passed #180352
test-mutter

00:03:54