LibWeb: Don't extrapolate transition properties for unknown properties

If we don't recognize a given transition-property value as a known CSS
property (one that we know about, not necessarily an invalid one),
we should not extrapolate the other transition-foo values for it.

Fixes #1480

Tests/LibWeb/Text/expected/WebAnimations/transition-unknown-property.txt

@@ -0,0 +1 @@
+   PASS (didn't crash)

Tests/LibWeb/Text/input/WebAnimations/transition-unknown-property.html

@@ -0,0 +1,15 @@
+<style>
+#foo {
+    transition-property: filter;
+    transition-timing-function: linear;
+    transition-duration:.3s;
+}
+</style>
+<div id="foo"></div>
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        foo.offsetWidth;
+        println("PASS (didn't crash)");
+    });
+</script>