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

**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:

```javascript
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

Start by running the provided reproduction through mergeTypeDefs and inspect the directive-argument merging path. Done means the list-coercion example completes without the TypeError and preserves the expected merged directive arguments.

Written by the indexing model from the issue text.