fix: support empty postgres enum variants

[leno23]

Summary

• generate a valid Rust enum variant when a Postgres enum contains an empty string value
• reuse the same variant-name fallback in both custom type and SQL conversion generation
• add regression tests covering the empty Postgres enum variant case

Fixes #289.

Test Plan

• cargo fmt --check
• cargo test -p cornucopia -- codegen::types::tests
• cargo test -p cornucopia
• cargo clippy -p cornucopia -- -D warnings
• git diff --check

[LouisGariepy]

Thanks for opening this PR. I added some thoughts here, in addition to what beanpuppy has already said. Please let me know what you think :)