[FIR] Add synthetic else branch to when if it's missing to LightTreeFirBuilder

compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/converter/ExpressionsConverter.kt

@@ -497,6 +497,7 @@ class ExpressionsConverter(
             if (hasSubject) {
                 subject.bind(this)
             }
+            var thereIsElse = false
             for (entry in whenEntries) {
                 val branch = entry.firBlock
                 branches += if (!entry.isElse) {
@@ -508,11 +509,15 @@ class ExpressionsConverter(
                         FirWhenBranchImpl(null, firCondition, branch)
                     }
                 } else {
+                    thereIsElse = true
                     FirWhenBranchImpl(
                         null, FirElseIfTrueCondition(null), branch
                     )
                 }
             }
+            if (!thereIsElse) {
+                branches += FirWhenBranchImpl(null, FirElseIfTrueCondition(null), FirEmptyExpressionBlock())
+            }
         }
     }