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

compiler/testData/diagnostics/tests/smartCasts/kt6819.fir.kt

@@ -0,0 +1,7 @@
+public fun test(o: String?): Boolean {
+    return when {
+        // Data flow info should propagate from o == null to o.length
+        o == null, o.length == 0 -> false
+        else -> true
+    }
+}