Using "null" YAML keyword as setting in overrides is problematic #570

These settings will result in an invalid config:

...
overrides:
  - go_type: "encoding/json.RawMessage"
    db_type: "json"
  - db_type: "uuid"
    null: true
    go_type: "*github.com/google/uuid.UUID"

The reason is tha null is a Keyword in YAML. However, this would be fine if I simply got an error using this invalid config. But I don't. instead the null value above is silently ignored and will always default to false.

I can fix the issue by quoting 'null' like this:

...
overrides:
  - go_type: "encoding/json.RawMessage"
    db_type: "json"
  - db_type: "uuid"
    'null': true
    go_type: "*github.com/google/uuid.UUID"

However, I think this setting name should be deprecated and something like nullable being used instead, and printing some type of warning or similar

https://github.com/kyleconroy/sqlc/blob/fa7660c700757b2654084a0c1a6ad227b7376b42/internal/config/config.go#L142-L143