LibJS: Call Array.prototype.findIndex() callback for empty elements

If the array value at the current index is empty, the callback will
be called with undefined as value.

Libraries/LibJS/Runtime/ArrayPrototype.cpp

@@ -489,12 +489,12 @@ Value ArrayPrototype::find_index(Interpreter& interpreter)
     auto array_size = array->elements().size();
 
     for (size_t i = 0; i < array_size; ++i) {
-        if (i >= array->elements().size())
-            break;
-
-        auto value = array->elements().at(i);
-        if (value.is_empty())
-            continue;
+        auto value = js_undefined();
+        if (i < array->elements().size()) {
+            value = array->elements().at(i);
+            if (value.is_empty())
+                value = js_undefined();
+        }
 
         MarkedValueList arguments(interpreter.heap());
         arguments.append(value);

Libraries/LibJS/Tests/Array.prototype.findIndex.js

@@ -23,6 +23,26 @@ try {
     assert(array.findIndex(value => value > 100) === -1);
     assert([].findIndex(value => value === 1) === -1);
 
+    var callbackCalled = 0;
+    var callback = () => { callbackCalled++; };
+
+    [].findIndex(callback)
+    assert(callbackCalled === 0);
+
+    [1, 2, 3].findIndex(callback);
+    assert(callbackCalled === 3);
+
+    callbackCalled = 0;
+    [1, , , "foo", , undefined, , ,].findIndex(callback);
+    assert(callbackCalled === 8);
+
+    callbackCalled = 0;
+    [1, , , "foo", , undefined, , ,].findIndex(value => {
+        callbackCalled++;
+        return value === undefined;
+    });
+    assert(callbackCalled === 2);
+
     console.log("PASS");
 } catch (e) {
     console.log("FAIL: " + e);

Libraries/LibJS/Tests/array-shrink-during-find-crash.js

@@ -17,7 +17,7 @@ try {
         callbackCalled++;
         a.pop();
     });
-    assert(callbackCalled === 3);
+    assert(callbackCalled === 5);
 
     console.log("PASS");
 } catch (e) {