Adding NULL to a date fails on Snowflake #441

A definition that adds NULL to a date fails on Snowflake. created + NULL compiles to "created" + NULL, and Snowflake rejects it: Invalid argument types for function '+': (DATE, NULL).

The type checker reads the expression as a temporal shift, because an untyped NULL is accepted as the interval, and gives it the type datetime. The emitters rewrite a temporal shift only when an operand has the type interval, so this one falls through to a plain +. Both packages have the same fault: _temporal_shift() and definition_sql_temporal_shift().

Found by running the corpus definition nullable date shift on Snowflake, with the live tests in #434.