AuthService.randomString: Throw Error instead of returning null

A string with a negative length is invalid, so we should throw here
instead of complicating the type with a possible null return value.

Signed-off-by: David Mehren <git@herrmehren.de>

src/auth/auth.service.ts

@@ -64,7 +64,7 @@ export class AuthService {
 
   randomString(length: number): Buffer {
     if (length <= 0) {
-      return null;
+      throw new Error('randomString cannot have a length < 1');
     }
     return randomBytes(length);
   }