Fixes #428: Handle None values in ChangeDiff diff properties

[jnovinger]

Fixes: #428

Fixes AttributeError when viewing the Diff tab for objects created or deleted in a branch.

Adds None guards to ChangeDiff properties that call .items() on potentially None fields:

• original_diff - returns {} when original is None (CREATE actions)
• modified_diff - returns {} when modified is None (DELETE actions)
• current_diff - returns {} when current is None (branch-created objects)
• altered_in_modified - returns set() when modified or original is None
• altered_in_current - returns set() when current or original is None

Scenarios fixed:

• CREATE: original=None, modified={...}, current=None
• DELETE: original={...}, modified=None, current={...}
• UPDATE on branch-created object: original={...}, modified={...}, current=None

[jeremystretch]

@jnovinger could you expand on the reproduction steps in #428? I'm not sure how to reproduce the bug before verifying the fix. Does this involve netbox-changes as well?

1. Create a branch and activate it
2. Create or update any object with custom fields (e.g., circuit termination, device)
3. View the Changes tab in the change request
4. Or attempt to revert the branch