fix: Allow Image.MAX_IMAGE_PIXELS to be None

[fsbraun]

Description

Related resources

We discourage to do this, but PIL.Image.MAX_IMAGE_PIXELS can be None. If so, please specify FILE_MAX_IMAGE_SIZE in your settings.py. If both are not set, any admin user can upload images as big as they wish opening up any project to memory bombs.

This PR allows PIL.Image.MAX_IMAGE_PIXELS to be None. If both PIL.Image.MAX_IMAGE_PIXELS and settings.FILER_MAX_IMAGE_PIXELS are not set, the checks framework will raise a warning.

• PIL.Image.MAX_IMAGE_PIXELS = None will trigger an exception in django-filer #1474
• #...

Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on
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