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

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

@@ -0,0 +1,14 @@
+enum class MyEnum {
+    A, B
+}
+
+fun foo(x: MyEnum?): Int {
+    val y: Int
+    // See KT-6046: y is always initialized
+    when (x) {
+        MyEnum.A -> y = 1
+        MyEnum.B -> y = 2
+        null -> y = 0
+    }
+    return y
+}