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

compiler/testData/diagnostics/tests/when/NonExhaustiveWithNullabilityCheck.fir.kt

@@ -0,0 +1,12 @@
+// KT-7857: when exhaustiveness does not take previous nullability checks into account
+enum class X { A, B }
+fun foo(arg: X?): Int {
+    if (arg != null) {
+        return when (arg) {
+            X.B -> 2
+        }
+    } 
+    else {
+        return 0
+    }
+}