[NI] Preserve nullability of resulting type from CST if it's possible

Consider common supertype of `S` and `Nothing`, where `S` has nullable
 upper bound or it's flexible. Before the fix, result was `S?`, which
 is correct but too conservative. Now, we'll preserve nullability of
 resulting type if it's already nullable.

 This happened because we were failing to find path of not-nullable
 types from `Nothing` to `S`, which should obviously exists by
 semantics of Nothing

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -9823,6 +9823,11 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                     runTest("compiler/testData/diagnostics/tests/inference/commonSystem/boundOnNullableVariable.kt");
                 }
 
+                @TestMetadata("cstFromNullableChildAndNonParameterizedType.kt")
+                public void testCstFromNullableChildAndNonParameterizedType() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/inference/commonSystem/cstFromNullableChildAndNonParameterizedType.kt");
+                }
+
                 @TestMetadata("dontCaptureTypeVariable.kt")
                 public void testDontCaptureTypeVariable() throws Exception {
                     runTest("compiler/testData/diagnostics/tests/inference/commonSystem/dontCaptureTypeVariable.kt");

compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java

@@ -9818,6 +9818,11 @@ public class DiagnosticsUsingJavacTestGenerated extends AbstractDiagnosticsUsing
                     runTest("compiler/testData/diagnostics/tests/inference/commonSystem/boundOnNullableVariable.kt");
                 }
 
+                @TestMetadata("cstFromNullableChildAndNonParameterizedType.kt")
+                public void testCstFromNullableChildAndNonParameterizedType() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/inference/commonSystem/cstFromNullableChildAndNonParameterizedType.kt");
+                }
+
                 @TestMetadata("dontCaptureTypeVariable.kt")
                 public void testDontCaptureTypeVariable() throws Exception {
                     runTest("compiler/testData/diagnostics/tests/inference/commonSystem/dontCaptureTypeVariable.kt");