zjquery: Support empty find results.

frontend_tests/node_tests/zjquery.js

@@ -90,6 +90,10 @@ run_test('finding_related_objects', () => {
     update_message_emoji('foo.png');
     assert.equal(emoji.attr('src'), 'foo.png');
 
+    // Sometimes you want to deliberatly test paths that do not find an
+    // element. You can pass 'false' as the result for those cases.
+    emoji.set_find_results('.random', false);
+    assert.equal(emoji.find('.random').length, 0);
     /*
     An important thing to understand is that zjquery doesn't truly
     simulate DOM.  The way you make relationships work in zjquery

frontend_tests/zjsunit/zjquery.js

@@ -199,6 +199,12 @@ exports.make_new_elem = function (selector, opts) {
             if (child) {
                 return child;
             }
+            if (child === false) {
+                // This is deliberately set to simulate missing find results.
+                // Return an empty array, the most common check is
+                // if ($.find().length) { //success }
+                return [];
+            }
             if (opts.silent) {
                 return self;
             }