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/testData/diagnostics/tests/smartCasts/smartcastToNothingAfterCheckingForNull.fir.kt

@@ -19,11 +19,11 @@ fun g(x: B<Int>) {
     val y = x.content
     if (y == null) {
         f(y)
-        g(y)
+        <!NONE_APPLICABLE!>g<!>(y)
     }
 
     if (y is Nothing?) {
         f(y)
-        g(y)
+        <!NONE_APPLICABLE!>g<!>(y)
     }
 }