Nullify blank credentials in the dependency proxy settings

🔭 Context

Follow up of Fix blank values handling in the dependency proxy (!140673 - merged).

Read the context there.

The issue with that MR is that the frontend could send mixed blank values (see #410726 (comment 1717461021)).

The solution here is to convert all blank values to nil either in the frontend or the backend. Given that this is used in a GraphQL endpoint, it's better to do it on the backend side to ensure behavior coherence no matter which client is using the endpoint.

Related to Maven dependency proxy: refine model validations (#436375 - closed)

🤔 What does this MR do and why?

• In the dependency proxy for packages settings model, nullify the credentials values if blank.
• Update related specs

🏎 MR acceptance checklist

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⚙ How to set up and validate locally

See !140673 (merged)