Make width/height optional in MediaAttachment's meta.original

[bocops]

Description

The following attributes are now optional/nullable, because they might not exist depending on the type of media attachment:

• meta.original.height
• meta.original.width
• meta.small.height
• meta.small.width

Closes #660.

Type of Change

• Bug fix

Breaking Changes

Consumers no longer can expect the attributes listed above to always exist / be non-null.

Mandatory Checklist

• I ran gradle check and there were no errors reported
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• I have added KDoc documentation to all public methods