Check if response could get generated

`json_encode` fails hard on PHP >= 5.5 if a non UTF-8 value is specified by returning false. Older PHP versions just nullify the value which makes it at least somewhat usable.

This leads to very confusing errors which are very hard to debug since developers are usually not aware of this. In this case I'd consider throwing a fatal exception – since it arguably is an error situation – is a fair solution since this makes developers and administrators aware of any occurence of the problem so that these bugs can get fixed.

Fixes https://github.com/owncloud/core/issues/17265