Do not annotate Kotlin members returning platform types as @Nullable

This leads to pointless warning in Java code

compiler/testData/asJava/lightClasses/nullabilityAnnotations/PlatformTypes.java

@@ -0,0 +1,11 @@
+public final class PlatformTypes implements kotlin.jvm.internal.KObject {
+    public final java.lang.String simplyPlatform() { /* compiled code */ }
+
+    @org.jetbrains.annotations.Nullable
+    public final java.util.List<java.lang.String> bothNullable() { /* compiled code */ }
+
+    @org.jetbrains.annotations.NotNull
+    public final java.util.List<java.lang.String> bothNotNull() { /* compiled code */ }
+
+    public PlatformTypes() { /* compiled code */ }
+}

compiler/testData/asJava/lightClasses/nullabilityAnnotations/PlatformTypes.kt

@@ -0,0 +1,9 @@
+// PlatformTypes
+
+import java.util.Collections
+
+class PlatformTypes {
+    fun simplyPlatform() = Collections.singletonList("")[0]
+    fun bothNullable() = Collections.emptyList<String>() ?: null
+    fun bothNotNull() = Collections.emptyList<String>()!!
+}