Pattern vs. nullable

Hello the community,

I got this schema definition in my OpenAPI 3.0.3 Yaml file:

username:
   type: string
   pattern: "^[a-zA-Z]{10}$"
   example: John

For some reason, I can't use allOf but I need to support a possible null value.

So I wrote:

username:
    type: string
    pattern: "^[a-zA-Z]{10}$"
    nullable: true
    example: John

Does it make sense to add nullable: true since I can change the regex to include null/empty value? (i.e.: '^[a-zA-Z]{3}$|^$')

Will "nullable" contradict the pattern, if it does not include null value as well?

Without allOf, how the pattern regex and the sub type "nullable" are connected?

Thank you for your comments