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

compiler/testData/diagnostics/tests/InferNullabilityInThenBlock.fir.kt

@@ -0,0 +1,9 @@
+fun ff(a: String) = 1
+
+fun gg() {
+    val a: String? = ""
+
+    if (a != null) {
+        ff(a)
+    }
+}