Fixes namespace_id for user with no groups import

What does this MR do?

User imports fail when the user is not part of a group, related to !20405 (merged) when we've removed a duplicate hidden_field_tag though I've missed that the original (remaining) hidden_field_tag was incorrect, this fixes it.

Related to #39078 (closed).

Screenshots

Because the current hidden_field_tag passes the value as value: id, it is reflected in the UI like this, invalid:

After this MR, it changes to what should be expected:

Does this MR meet the acceptance criteria?

Conformity

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