Fix validation of project_key

What does this MR do?

!24063 (merged) added a new project_key attribute to service desk settings. This attribute will be used in future to customize incoming email address, but it's optional and can be left empty.

Because it can be left empty, we should skip uniqueness validation if the attribute is not set.

This is a regression because now service desk settings form can not be saved because it will always fail on validation that project key has already been taken.

Screenshots

Does this MR meet the acceptance criteria?

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