[FE 1.0] Don't use identifier in WhenMissingCase.kt if it wasn't provided

^KT-49702 Fixed

compiler/testData/diagnostics/tests/when/kt49702.kt

@@ -0,0 +1,9 @@
+// FIR_IDENTICAL
+sealed class A {
+    class B:A()
+    class<!SYNTAX!><!> :A()
+    fun test(a : A) {
+        <!NON_EXHAUSTIVE_WHEN_STATEMENT("sealed class/interface; 'is B', 'is <no name provided>' branches or 'else' branch instead")!>when<!>(a) {
+        }
+    }
+}