doesnotcontain should include null and empty values

[corydeppen]

When the doesnotcontain operator is used, ToExpression is currently just using !{field}.{comparison}(@{index}). This leaves out rows where the field value is null or empty, though. Checking the field for null or empty should produce a more accurate query and result. I'd be happy to submit a quick PR if you agree.

[burkeholland]

Can you provide a specific test case where the results are not as desired? On a high level, I understand the issue, but it would be nice to have a more concrete example where something would fail.

[corydeppen]

For example, when trying to look at rows where descriptions don't contain "brake", the SQL that is generated by EF looks like (NOT([Extent1].[PartDescription] LIKE '%brake%')). This will return rows that have a string value that doesn't contain "brake", as well as rows where the value is an empty string, but will not include nulls because NULL is NOT LIKE and NOT NOT LIKE.