sql.py: run_query(): Only pass non-None debug_msg_ref to DbConn.run_query() if filtering with log_ignore_excs or can't mogrify() (and therefore can't print queries before they're run)

sql.py: DbConn: Added can_mogrify() and use it in mogrify()

sql.py: DbConn.run_query(): Log query before running if no debug_msg_ref specified. Documented debug_msg_ref param.

sql.py: DbConn: Added mogrify() and use it in esc_value()

schemas/functions.sql: _nullIf(): Fixed bug where wrong var name was used to retrieve type param. Reenabled _nullIf now that it's working.

sql_gen.py: ColDict.__setitem__(): Translate NULL values to the appropriate default value of the key column (which is often NULL, but not always) so that select query filtering/joins work correctly

sql.py: DbConn.col_default(): Fixed bug where returned string needed to be wrapped in sql_gen.as_Code() because it's sometimes a SQL expression and sometimes NULL

sql_gen.py: Added as_Code(). Split SQL code objects into separate sections so unparameterized classes would be separate from general classes.

sql_gen.py: Code.to_str(): Fixed bug where needed to raise NotImplementedError, not NotImplemented

dicts.py: is_dict(): Fixed bug where lists also have getitem() methods, so keys() was checked for instead