GH-2560 - Fix warnings in Kotlin test-classes.

Closes #2560.

Author: michael-simons

src/test/kotlin/org/springframework/data/neo4j/integration/imperative/KotlinInheritanceIT.kt

@@ -229,7 +229,7 @@ class KotlinInheritanceIT @Autowired constructor(
 			return BookmarkCapture()
 		}
 
-		override fun getMappingBasePackages(): Collection<String>? {
+		override fun getMappingBasePackages(): Collection<String> {
 			return setOf(Inheritance::class.java.getPackage().name)
 		}
 

src/test/kotlin/org/springframework/data/neo4j/integration/imperative/KotlinProjectionIT.kt

@@ -127,7 +127,7 @@ internal class KotlinProjectionIT {
 		/**
 		 * Make sure that particular entity is know. This is essential to all tests GH-2349
 		 */
-		override fun getMappingBasePackages(): Collection<String>? {
+		override fun getMappingBasePackages(): Collection<String> {
 			return listOf(DepartmentEntity::class.java.getPackage().name)
 		}