Fix entity saving when '' is used as a primary key value.

Having '' as the primary key value is almost never wanted as '' is cast
to null or 0. Instead filter out this value and treat it as a mistake.

Refs #4152
Move blank primary key filtering to the marshaller.

I agree with jose_zap that having it here makes more sense, as the
marshaller is responsible for converting request data into sensical
entities.
Merge pull request #4242 from cakephp/3.0-table-save-id

Fix entity saving when '' is used as a primary key value.