fix comparing content for a "simple" field that already has an empty value

[i-just]

Description

If you’re importing into a feed with setEmptyValues turned on, compareContent set to true (default), you have an existing element with a field that’s already empty, and you’re importing an empty value into that field, the content comparison was incorrectly reporting that the content has changed.

This happened because the Hash::check will always return false if the key exists but the value is null, causing the content to never actually get compared.

(Hash::extract will return an empty array causing Hash::check to return false.)

Related issues

#1615