Optimize out empty ifs

If an if statement has no true or false sub-statements, there is no point including the if statement in the LIR graph. So instead of constructing a sub-graph for the if statement — which would include an empty if vertex and nothing else — and returning it, we return an empty graph object.

Fixes elastic#9314

logstash-core/src/main/java/org/logstash/config/ir/imperative/IfStatement.java

@@ -84,6 +84,12 @@ public Graph toGraph() throws InvalidIRException {
         Graph trueGraph = getTrueStatement().toGraph();
         Graph falseGraph = getFalseStatement().toGraph();
 
+        // If there is nothing in the true or false sections of this if statement,
+        // we can omit the if statement altogether!
+        if (trueGraph.isEmpty() && falseGraph.isEmpty()) {
+            return new Graph();
+        }
+
         Graph.GraphCombinationResult combination = Graph.combine(trueGraph, falseGraph);
         Graph newGraph = combination.graph;
         Collection<Vertex> trueRoots = trueGraph.roots().map(combination.oldToNewVertices::get).collect(Collectors.toList());