[FIR-TEST] Add new testdata generated after changes in previous commit

compiler/testData/diagnostics/tests/smartCasts/varnotnull/varNull.fir.kt

@@ -0,0 +1,6 @@
+// !WITH_NEW_INFERENCE
+fun foo(): Int {
+    var s: String? = "abc"
+    s = null
+    return s.<!UNRESOLVED_REFERENCE!>length<!>
+}