CRM-13120 - APIv3 - Don't return "null" from "create" action

The patch post-processes API results to convert 'null' to '' (using NullOutputCoder). 

Note that the PR looks lengthy because (a) I wanted to reuse code from the old HTMLInputCoder -- the shared code moved to a new class, AbstractFieldCoder, and (b) I put the three classes (AbstractFieldCoder, HTMLInputCoder, NullOutputCoder) into a new namespace (CRM_Utils_API).

The old CRM_Core_HTMLInputCoder is left as a stub for compatibility with packages/HTML/QuickForm.php.