Ensure access_level is present before comparison

What does this MR do and why?

This commit addresses an ArgumentError exception in Sentry caused by comparing an Integer with nil in the Member#higher_access_level_than_group validation:

ArgumentError
comparison of Integer with nil failed (ArgumentError)

    if highest_group_member && highest_group_member.access_level > access_level
                                                                   ^^^^^^^^^^^^

The validation assumes access_level is present, but this isn't guaranteed. Although we have an access_level_inclusion validation to ensure access_level is not nil, the order of validations in Rails is not predetermined.

To resolve this, we modify the validation to work even when some attributes are nil, ensuring robustness regardless of validation order.

References

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• Sentry link

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How to set up and validate locally

Run the newly added specs on the master branch.

Related to #507270 (closed)