Fix end unconditionally decrementing the split level, but case not incrementing it in a plain select

[lorenzhs]

Fixes inconsistent level handling of CASE ... END:

• CASE only sets the _in_case flag and returns a +1 level diff if it's in a CREATE statement
• the matching END unconditionally decreases the level

This led to the level being one lower than it should have been after the CASE WHEN ... END block.

The way this caused trouble was in splitting: combined with some escaping trouble (#838) this led to a query incorrectly being split into two.

Checklist:

• ran the tests (pytest)
• all style issues addressed (flake8)
• your changes are covered by tests
• your changes are documented, if needed