I need your help to create a trigger in order to validate if the “national ID” field

Dears,

I need your help to create a trigger in order to validate if the “national ID” field located in the contact object, is “not null” before converting any lead to opportunity, in other words, prevent changing any lead status to converted if the NID is null

I really appreciate your help thanks in advance

Fields API names:

Lead status API name: StatusCode

NID API Name: XX_NationalID_c