Minor fix for the User Resource and correction in find_attr

[aldrickdev]

What does this PR do?

1. Errors when syncing the User Resource because a new attribute was added to the user called mfa_enabled. This attribute is currently not documented as requested by the @DataDog/team-aaa-identity team, related PR.
2. Solves issue where the find_attr would not check if the variable r_obj is not None before performing operations on it, causing failures.

Description of the Change

For issue 1, I added the attribute mfa_enabled to be excluded so that it is ignored. For issue 2, we check that r_obj is not None before trying any operations.

Alternate Designs

Possible Drawbacks

Verification Process

I ran manual and automated tests to make sure that the issues don't happen again.

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.