Redundant Unit: fix false positive when return type is nullable Unit

#KT-30038 Fixed

idea/src/org/jetbrains/kotlin/idea/inspections/RedundantUnitExpressionInspection.kt

@@ -44,7 +44,8 @@ private fun KtReferenceExpression.isRedundantUnit(): Boolean {
     if (!isUnitLiteral()) return false
     val parent = this.parent ?: return false
     if (parent is KtReturnExpression) {
-        return parent.expectedReturnType()?.nameIfStandardType != KotlinBuiltIns.FQ_NAMES.any.shortName()
+        val expectedReturnType = parent.expectedReturnType() ?: return false
+        return expectedReturnType.nameIfStandardType != KotlinBuiltIns.FQ_NAMES.any.shortName() && !expectedReturnType.isMarkedNullable
     }
     if (parent is KtBlockExpression) {
         // Do not report just 'Unit' in function literals (return@label Unit is OK even in literals)

idea/testData/inspectionsLocal/redundantUnitExpression/returnAsNullableUnit.kt

@@ -0,0 +1,5 @@
+// PROBLEM: none
+
+fun foo(): Unit? {
+    return Unit<caret>
+}

idea/tests/org/jetbrains/kotlin/idea/inspections/LocalInspectionTestGenerated.java

@@ -5733,6 +5733,11 @@ public class LocalInspectionTestGenerated extends AbstractLocalInspectionTest {
         public void testReturnAsNullableAny() throws Exception {
             runTest("idea/testData/inspectionsLocal/redundantUnitExpression/returnAsNullableAny.kt");
         }
+
+        @TestMetadata("returnAsNullableUnit.kt")
+        public void testReturnAsNullableUnit() throws Exception {
+            runTest("idea/testData/inspectionsLocal/redundantUnitExpression/returnAsNullableUnit.kt");
+        }
     }
 
     @TestMetadata("idea/testData/inspectionsLocal/redundantVisibilityModifier")