Field null in Override configuration is ignored. #413

Hi, thank you for the nice software. I'm facing the following issue, in which the overrides settings was not applied properly.

When the configuration is:

overrides:
  - go_type: "sql.NullInt64"
    db_type: "integer"
    null: true

the null config value is not decoded properly. However, if I changed the configuration to (use "null" as field name)

overrides:
  - go_type: "sql.NullInt64"
    db_type: "integer"
    "null": true

sqlc will correctly override type as expected. In the former case, YAML decoder is failed to decode the configuration (the value is always false), perhaps because the field name is null.
Probably this issue is related to #412 too?