it's incorrect to zero out table->triggers->extra_null_bitmap
before a statement, because if insert uses an explicit field list
and omits a field that has no default value, the field should
get NULL implicitly. So extra_null_bitmap should have 1s for all
fields that have no defaults
* create extra_null_bitmap_init and initialize it as above
* copy extra_null_bitmap_init to extra_null_bitmap for inserts
* still zero out extra_null_bitmap for updates/deletes where
all fields definitely have a value
* make not_null_fields_have_null_values() to send
ER_NO_DEFAULT_FOR_FIELD for fields with no default and no value,
otherwise creation of a trigger with an empty body would change the
error message
