sql/schemachanger: handle empty schemas gracefully

[fqazi]

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.

[cockroach-teamcity]

This change is 

[fqazi]

@rafiss @annrpom TFTR

bors r+

[craig[bot]]

Build succeeded:

• linux_amd64_fips_build
• unit_tests
• docker_image_amd64
• linux_arm64_build
• local_roachtest_fips
• local_roachtest
• windows_build
• examples_orms
• macos_arm64_build
• macos_amd64_build
• check_generated_code
• lint
• linux_amd64_build
• acceptance

[blathers-crl[bot]]

Based on the specified backports for this PR, I applied new labels to the following linked issue(s). Please adjust the labels as needed to match the branches actually affected by the issue(s), including adding any known older branches.

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Issue #129676: branch-release-23.2, branch-release-24.1, branch-release-24.3.

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