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#### Describe the bug
Uncaught TypeError is raised for axios.create with axios version 0.22.0
Issue is observed while merging the defaultConfig, with instanceConfig.
We are setting instanceConfig only

#### To Reproduce
Exception is raised while creating axios instance with below sample code.


```js
const service  = axios.create({
     baseURL: "host",
     timeout: 3000
})

```

#### Expected behavior
Same works with axios version 0.21.4

#### Environment
 - Axios Version [0.22.0]
 - Browser [Chrome, Edge]
 - Browser Version [Version 94.0.4606.71 (Official Build) (64-bit)]
 - Node.js Version [14.15.1]
 - Additional Library Versions [VueJS 3.2.19, vue-axios 3.3.7]

#### Additional context/Screenshots
Exception stack trace
axios.js:1308 Uncaught TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
    at mergeConfig (axios.js:1308)
    at Function.create (axios.js:1712)

test/typescript/axios.ts
.then(handleResponse)
  .catch(handleError);
axios.get('/user', { params: { id: 12345 } })
  .then(handleResponse)
  .catch(handleError);
axios.head('/user')
  .then(handleResponse)
  .catch(handleError);
axios.options('/user')
  .then(handleResponse)
  .catch(handleError);
axios.delete('/user')
  .then(handleResponse)
  .catch(handleError);
axios.post('/user', { foo: 'bar' })
  .then(handleResponse)
  .catch(handleError);
axios.post('/user', { foo: 'bar' }, { headers: { 'X-FOO': 'bar' } })
  .then(handleResponse)
  .catch(handleError);
axios.put('/user', { foo: 'bar' })
  .then(handleResponse)
  .catch(handleError);
axios.patch('/user', { foo: 'bar' })
  .then(handleResponse)
  .catch(handleError);
interface UserCreationDef {
    name: string;
}
interface User {