Handle null values in yaml import

Problem/Motivation

The entityExists method doesn't properly check null values when building the query conditions. You end up with "LIKE NULL" in the where clause when it should be "IS NULL"

Steps to reproduce

I found this because we have a process that exports existing menu content items and sometimes the description or other properties are actually NULL. I could exclude the null values in my exporter, but I think we can handle this gracefully.

Proposed resolution

Create a separate condition when null is found.