Fix custom-error-definition to support constructors without a body

[pat841]

Using this example:

export class CustomError extends Error {
  type: string;
  text: string;
  reply?: any;
  constructor(type: string, text: string, reply?: any);
}

The error TypeError: Cannot read property 'body' of null gets thrown for the line const constructorBody = constructorBodyNode.body;.

We can fix this by returning early if no constructor body is available.

[sindresorhus]

Can you add a test?

[mf-pherlihy]

Can you add a test?

I tried but unfortunately the tests dont support the Typescript syntax.

The primary issue is constructor(type: string, text: string, reply?: any); not having a constructor body. This is supported by Typescript but throws A fatal parsing error occurred: Parsing error: Unexpected token ; in a non-TS environment.

[futpib]

@mf-pherlihy @sindresorhus I took the liberty of adding a typescript test, check it out.