Max Image Attachments: Default to integer

[obenland]

It's possible for the register_post_meta call to trigger a _doing_it_wrong() notice, when the default value is not of type integer.

Proposed changes:

• Adds unit test.
• Adds option callback to make sure it always returns an integer.
• Tightens sanitization to make sure it never saves an empty string.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• While on trunk, activate Classic Editor plugin to make the setting show up.
• Delete the number in the max image attachments setting and save.
• Make sure the input field is empty and a _doing_it_wrong() notice appears (DEBUG enabled).
• Switch to this branch. The notice should no longer appear.