source: Fix key-value load gen w/o primary export

[jkosh44]

This commit fixes the key-value load generator source when the primary
export is removed via the force_source_table_syntax flag.

Fixes #MaterializeInc/database-issues/issues/8904

Motivation

This PR fixes a recognized bug.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[aljoscha]

I'm not sure I like this, but here we are. 😅 (of course not because of anything you did wrong)

I think this is a good fix in our situation, though, and will help unblock the migration work needed for table-from-source syntax!