LibWeb: Null check fonts after parsing them in CRC2D.font assignment

Fixes an issue where setting CRC2D.font to an unparseable value would
assert due to a null dereference.

Tests/LibWeb/Text/input/canvas/basic.html

@@ -0,0 +1,11 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        let canvas = document.createElement("canvas");
+        let context = canvas.getContext("2d");
+        context.font = '20px SerenitySans';
+        println(context.font);
+        context.font = '!!!'; // Invalid value, should be ignored.
+        println(context.font);
+    });
+</script>