Add null check to animations for registered custom property initial values

third_party/WebKit/LayoutTests/animations/custom-properties/empty-initial-value-crash.html

+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<style>
+#target {
+  --x: initial;
+}
+</style>
+<div id="target"></div>
+<script>
+setup(() => {
+  CSS.registerProperty({name: '--x'});
+});
+
+test(() => {
+  var animation = target.animate({'--x': 'test'}, 1);
+  assert_equals(getComputedStyle(target).getPropertyValue('--x'), '');
+  animation.cancel();
+}, 'Do not crash when animating with an underlying value of initial');
+
+test(() => {
+  var animation = target.animate({'--x': 'initial'}, {fill: 'forwards'});
+  assert_equals(getComputedStyle(target).getPropertyValue('--x'), '');
+  animation.cancel();
+}, 'Do not crash when animating an empty initial value');
+
+test(() => {
+  var animation = target.animate({'--x': 'inherit'}, {fill: 'forwards'});
+  assert_equals(getComputedStyle(target).getPropertyValue('--x'), '');
+  animation.cancel();
+}, 'Do not crash when animating an inherited empty initial value');
+</script>