[FE 1.0] Check for type mismatch for empty when statements

^KT-47922 Fixed

compiler/testData/diagnostics/tests/sealed/WhenOnEmptySealed.fir.kt

@@ -0,0 +1,11 @@
+// !DIAGNOSTICS: -UNUSED_EXPRESSION
+sealed class Sealed {
+
+}
+
+fun foo(s: Sealed): Int {
+    return <!NO_ELSE_IN_WHEN!>when<!>(s) {
+        // We do not return anything, so else branch must be here
+    }
+}
+

compiler/testData/diagnostics/tests/sealed/WhenOnEmptySealed.kt

@@ -1,12 +1,11 @@
-// FIR_IDENTICAL
 // !DIAGNOSTICS: -UNUSED_EXPRESSION
 sealed class Sealed {
 
 }
 
 fun foo(s: Sealed): Int {
-    return <!NO_ELSE_IN_WHEN!>when<!>(s) {
+    return <!TYPE_MISMATCH!><!NO_ELSE_IN_WHEN!>when<!>(s) {
         // We do not return anything, so else branch must be here
-    }
+    }<!>
 }