Minor: Fix test data

idea/testData/codeInsight/overrideImplement/jdk8/overrideCollectionStream.kt.after

@@ -1,7 +1,7 @@
 import java.util.stream.Stream
 
 abstract class A : List<String> {
-    override fun stream(): Stream<String>? {
+    override fun stream(): Stream<String> {
         <selection><caret>return super.stream()</selection>
     }
 }