FIR DFA: smartcast variable to Nothing? on null assignment

In order to make resolution still work for members not available from
`Nothing`, we track the type without `Nothing?` and use that for
resolution instead.

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -466,6 +466,7 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
             +field("typesFromSmartCast", "Collection<ConeKotlinType>", null, customType = coneKotlinTypeType)
             +field("originalType", typeRef)
             +field("smartcastType", typeRef)
+            +field("smartcastTypeWithoutNullableNothing", typeRef)
             +smartcastStability
         }