Fix bug in image_util.py

[kostrykin]

Fix bug introduced in 3d3ad72 (#17556).

When running planemo test:

Traceback (most recent call last):
File "/home/void/miniconda3/envs/galaxy2024/bin/planemo", line 8, in
sys.exit(planemo())
File "/home/void/miniconda3/envs/galaxy2024/lib/python3.8/site-packages/click/core.py", line 1157, in call
return self.main(*args, **kwargs)
File "/home/void/miniconda3/envs/galaxy2024/lib/python3.8/site-packages/galaxy/util/image_util.py", line 20, in image_type
if Image is not None:
NameError: name 'Image' is not defined

We forgot to change PIL = None to Image = None. This is hereby done.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[nsoranzo]

Thanks for the fix @kostrykin !