Merging directive arguments from extensions doesn't consider list input coercion

[andreasdamm]

Describe the bug

When adding an already defined directive through an extension statement, it appears that mergeTypeDefs attempts to merge the arguments of duplicated directives if the original was of list type. If the extension directive relies on list input coercion by not specifying a list, an exception is thrown.

To Reproduce
Steps to reproduce the behavior:

const { mergeTypeDefs } = require('@graphql-tools/merge');
const { parse } = require('graphql');

const merged = mergeTypeDefs(
   parse(
      `
         directive @foo(x: [Int!]!) repeatable on SCALAR
 
         scalar Foo @foo(x: [1])
 
         extend scalar Foo @foo(x: 2)
     `
   )
);

Expected behavior

Script runs to completion successfully. Instead it ends in TypeError: Cannot read property 'filter' of undefined

Environment:

• OS: Windows
• @graphql-tools/merge: 8.2.6
• graphql: 16.3.0
• NodeJS: v14.17.6