TS Type cheking error

[juanem1]

Describe the problem

function getSigningKey() only accept string param, but null or undefined are valid for json' s with only one key.

What was the expected behavior?

TS should not show and error when calling getSigningKey() with no params or undefined param

Reproduction

async getPublicKey(kid?: string): Promise<string> {
    const client = jwksClient({
      cacheMaxAge: this.cacheMaxAge,
      jwksUri: this.url,
    });
    const key = await client.getSigningKey(kid); <- Error: string | undefined is not assignable to parameter of type 'string' 
    return key.getPublicKey();
}