Fix getIncludesDefault() for single default case

[utafrali]

Fixes #6666

The condition was only checking the second position for FakeDefaultLiteral when there were two expressions, so single-expression cases were being skipped. Updated it to check both positions and added a test to verify.

[SirYwell]

case default isn't valid Java code, it's either default (represented by an empty getCaseExpressions() list) or case null, default (represented by getIncludesDefault() returning true). The reason for the getIncludesDefault() method is that we cannot include the default as a case expression, but need to remember it in a different way. That's only necessary for case null, default, therefore it does not cover default. In other words, case null, default includes default in its case expressions, while default does not have any case expressions, and therefore does not include default.