fix(format): allow undefined decimals

[oumar-fall]

The goal is to allow the format operator to accept nullable decimals in case where value is itself nullable.

In this way, the typing is:

• format(bigint, number): string
• format(bigint | undefined, number): string | undefined
• format(bigint | null, number): string | null
• format(bigint | undefined | null, number): string | undefined | null
• format(bigint, number | undefined): string | undefined
• format(bigint, number | null): string | null
• format(bigint, number | undefined | null): string | undefined | null
• format(bigint | undefined, number | null): string | undefined | null
• format.default(string).(bigint | undefined | null, number | undefined | null): string