When `ActiveRecord::Coders::JSON` serialization is used and the default of the column returns `''` it raises the following error:

```
JSON::ParserError: A JSON text must at least contain two octets!
```

If MySQL is running in non-strict mode, it returns an empty string as column default for a text column:

```ruby
def extract_default
  if blob_or_text_column?
    @default = null || strict ? nil : ''
  end
end
```

Since `''` is invalid JSON, there shouldn't be an attempt to parse it, it should be treated like nil.
ActiveRecord::Coders::JSON should behave consistently for all possible non-user-set column default values.