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

compiler/testData/diagnostics/tests/smartCasts/loops/whileNull.fir.kt

@@ -0,0 +1,5 @@
+fun foo(s: String?): Int {
+    while (s==null) {
+    }
+    return s.<!INAPPLICABLE_CANDIDATE!>length<!>
+}