xyz grid handle axis_type is None

[w-e-w]

Description

xyz grid
if the selected axle from dropdown is not found the axis_type index will be None
triggering and error

  File "B:\GitHub\stable-diffusion-webui\scripts\xyz_grid.py", line 479, in select_axis
    choices = self.current_axis_options[axis_type].choices
TypeError: list indices must be integers or slices, not NoneType

simple fix
if axis_type is None then set to index 0 (Nothing)

same logic also applies to run()

Screenshots/videos:

2023-12-22.00_14_11_609.chrome.mp4

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests