[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) {
+        }
+    }
+}

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

@@ -0,0 +1,25 @@
+package
+
+public sealed class A {
+    protected constructor A()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final fun test(/*0*/ a: A): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public final class <no name provided> : A {
+        public constructor <no name provided>()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun test(/*0*/ a: A): kotlin.Unit
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public final class B : A {
+        public constructor B()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun test(/*0*/ a: A): kotlin.Unit
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}