Fix unsetting credentials data for pull mirrors

Merged Nick Thomas requested to merge (removed):fix-pull-mirror-setting-nil into master

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?

What are the relevant issue numbers?

Closes #3308 (closed)

Edited by Nick Thomas