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

compiler/testData/diagnostics/tests/cast/NullableToNullable.fir.kt

@@ -0,0 +1,7 @@
+// From KT-13324: always succeeds
+val x = null as String?
+// From KT-260: sometimes succeeds
+fun foo(a: String?): Int? {
+    val c = a as? Int?
+    return c
+}