fix: update empty-else merge test to use ExpressionCase after #492

PR #492 changed boolean IF-branch flows from EnumerationCase to
ExpressionCase. The regression test was asserting on the old type directly;
switch to flowCaseString() to handle all CaseValue variants uniformly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ako

mdl/executor/cmd_microflows_builder_empty_else_merge_test.go

@@ -53,7 +53,7 @@ func TestIfEmptyElseBodyWithContinuingThenEmitsFalseFlowToMerge(t *testing.T) {
 		if flow.OriginID != split.ID || flow.DestinationID != merge.ID {
 			continue
 		}
-		if ec, ok := flow.CaseValue.(microflows.EnumerationCase); ok && ec.Value == "false" {
+		if flowCaseString(flow.CaseValue) == "false" {
 			hasFalseFlow = true
 		}
 	}