OGM-618 Don't store null in association rows for document stores
OGM-618 Don't store null in association rows for map-based stores
OGM-618 Using more descriptive method name
OGM-618 Don't store null in tuples in map-based dialects
OGM-618 Don't store null values in entity documents for MongoDB
Fix missing break; in MapHelpers
OGM-618 Treat associationRow PUT_NULL like REMOVE

In practice the test suite does not reach PUT_NULL in that case
OGM-618 Do not put null values in CouchDBDocument
OGM-618 Remove dotted columns when set to null in MongoDB