bug: possible UnboundError on model_clims_changed

[tlambert03]

See UnboundLocalError above for original traceback.
╭─────────────────────────────── Traceback (most recent call last) ────────────────────────────────╮
│ /Users/talley/dev/self/ndv/.venv/lib/python3.12/site-packages/psygnal/_signal.py:1196 in         │
│ _run_emit_loop                                                                                   │
│                                                                                                  │
│ /Users/talley/dev/self/ndv/.venv/lib/python3.12/site-packages/psygnal/_signal.py:1225 in         │
│ _run_emit_loop_immediate                                                                         │
│                                                                                                  │
│ /Users/talley/dev/self/ndv/.venv/lib/python3.12/site-packages/psygnal/_weak_callback.py:453 in   │
│ cb                                                                                               │
│                                                                                                  │
│ /Users/talley/dev/self/ndv/src/ndv/controllers/_channel_controller.py:63 in                      │
│ _on_model_clims_changed                                                                          │
│                                                                                                  │
│    60 │   │   │   min_max = clims.calc_clims(handle.data())                                      │
│    61 │   │   │   handle.set_clims(min_max)                                                      │
│    62 │   │   for v in self.lut_views:                                                           │
│ ❱  63 │   │   │   v.set_clims_without_signal(min_max)                                            │
│    64 │   │   │   v.set_auto_scale_without_signal(is_autoscale)                                  │
│    65 │                                                                                          │
│    66 │   def _on_model_gamma_changed(self, gamma: float) -> None:                               │
╰──────────────────────────────────────────────────────────────────────────────────────────────────╯
UnboundLocalError: cannot access local variable 'min_max' where it is not associated with a value

hit this while playing with #105 ... but it's definitely a bug in main that I think I introduced in #94
looks like it will happen when there are no canvas image handles, but there is a lut_view widget (which I suppose could happen briefly during channel mode changing)