Remove redundant null check in NodeVisitor

A `ConditionalExpression`, differing from an `IfStatement`, always has
an `else` case.

Author: fmeum

src/main/java/net/starlark/java/syntax/NodeVisitor.java

@@ -174,9 +174,7 @@ public void visit(@SuppressWarnings("unused") Comment node) {}
   public void visit(ConditionalExpression node) {
     visit(node.getCondition());
     visit(node.getThenCase());
-    if (node.getElseCase() != null) {
-      visit(node.getElseCase());
-    }
+    visit(node.getElseCase());
   }
 
   // methods dealing with sequences of nodes