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

compiler/testData/diagnostics/tests/controlFlowAnalysis/kt6788.fir.kt

@@ -0,0 +1,10 @@
+class A(val next: A? = null) {
+    val x: String
+    init {
+        next?.x = "a"
+    }
+}
+
+class B(val next: B? = null) {
+    var x: String = next?.x ?: "default" // it's ok to use `x` of next
+}