Fix EZP-23499: Fatal error removing class with unexistant datatype.

JIRA: https://jira.ez.no/browse/EZP-23499

This prevents a PHP fatal error when removing a class (contentType) if a datatype has been removed.
Instead, it is logged to error.log

sounds good, thanks.
I'll update to a strict check for simplicity, because this (actually, `eZDataType::create()`) should always return either a datatype or `null`.

(Even considering the possibility of loading an instance of some other class, a PHP fatal error would be appropriate in that case, I believe :) )