Add Mapping for General Insert and Update that Renders null Values More Simply

[jeffgbutler]

Rather than rendering null values as a null parameter, the new mapping will simply render null values as "null" in the generated SQL.

This mapping will be important in the updated Kotlin DSL where we more clearly express nullability expectations.

[coveralls]

Coverage remained the same at 100.0% when pulling d84743e on jeffgbutler:value-or-null-mapping into b1fa3a6 on mybatis:master.