Add support for column filters with `null` value

[dmitrykuzmin]

Currently, we support @Nullable entity columns which can have null as their value, but do not support querying for null via column filters.

ColumnFilters utility responsible for filter creation currently does not accept null as filter value and nor do querying mechanisms of in-memory storage, JDBC storage and Google Cloud Storage.

We should add null support for EQUALS operator or even create a new separate operator.