release-23.2: sql/schemachanger: handle empty schemas gracefully

[blathers-crl]

Backport 1/1 commits from #135910 on behalf of @fqazi.

/cc @cockroachdb/release

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Previously, when running CREATE SCHEMA in the declartive schema changer empty schema names with end up with an assertion failure error. This was because the declarative schema change code did not handle these cases properly. When an empty schema is specified ideally a syntax error should be generated. To address this, this patch will generate the correct errors and adds logic tests.

Fixes: #129676

Release note (bug fix): CREATE SCHEMA now returns the correct error if a the schema name is missing.

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Release justification: low risk fix for a bug that produces incorrect errors

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
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[cockroach-teamcity]

This change is