AssertEqualsNullToAssertNull: support Kotlin via LST manipulation

[timtebeek]

AssertEqualsNullToAssertNull used a JavaTemplate, which can only emit Java, so it crashed with "Expected a template that would generate exactly one statement to replace one statement, but generated 0" when the assertEquals(a, null) call was in a Kotlin source (e.g. assertEquals(values[0], null)).

This reworks the recipe to transform via direct LST manipulation instead — renaming assertEquals to assertNull, dropping the null argument, resolving the correct assertNull overload, and updating imports — which works uniformly for Java and Kotlin. Java behavior is unchanged; a new Kotlin test covers all four argument shapes plus the assertEquals to assertNull import rewrite.