Add null check to animations for registered custom property initial values

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

Index: third_party/WebKit/LayoutTests/animations/custom-properties/empty-initial-value-crash.html
diff --git a/third_party/WebKit/LayoutTests/animations/custom-properties/empty-initial-value-crash.html b/third_party/WebKit/LayoutTests/animations/custom-properties/empty-initial-value-crash.html
new file mode 100644
index 0000000000000000000000000000000000000000..efd5e5d0053e795404e94c9f9a3ed506715b82b6
--- /dev/null
+++ b/third_party/WebKit/LayoutTests/animations/custom-properties/empty-initial-value-crash.html
@@ -0,0 +1,31 @@
+<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>