Fix unsetting credentials data for pull mirrors

What does this MR do?

Removes non-present? attributes from the project import data credentials hash instead of setting the value to "" or nil.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Fix a regression in %9.5

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #3308 (closed)