Allow null indexes on formSerializer and paramsSerializer v0.x

[WillianAgostini]

Fixes #4959
Docs: automatic-serialization-to-formdata

Allow indexes to receive null value.

axios.create({
  headers: {
    'Content-Type': 'application/json',
  },
  formSerializer: {
    indexes: null,
  },
  paramsSerializer: {
    indexes: null,
  },
});

Axios Version: v0.x

[adamsuskin]

@jasonsaayman can we tag a new hotfix version for this 0.27.3?

[nancy-alpha]

@jasonsaayman @adamsuskin I'm also interested in a 0.27.3 version with this fix, if it's possible!