WTF::dynamic_cf_cast<> should not assert in Debug builds.

The proper way to use dynamic_cf_cast<> is to do a nullptr check on its return results, and take action if the cast failed.  One of the most common use cases is when decoding (untrusted) serialized data, and instead of crashing, we just want to fail decoding on that particular chunk serialized data.

In some cases, we may way to add a nullptr check before calling it to differentiate that scenario from the wrong object type being detected.

We never want to use dynamic_cf_cast<> and assume it always succeeds, though.  In that case, we either want to use checked_cf_cast<> instead, or add a nullptr check to handle that scenario.

<rdar://problem/73451079>