fix(addresses): payment_cred stake_address: null

[1000101]

Previously, first queried stake address was displayed. But since tthere can be multiple stake addresses for a given payment_cred, this information was not correct. The fix is to always display null instead of a random stake_address.

Testing vector:
http://localhost:3000/addresses/addr_vkh1vt2ntv45rx6e7rfk2qfw2434kdnl0k8fl4h3h6p55d2j6k6czz3

Current state:

{
  "address": "addr_vkh1vt2ntv45rx6e7rfk2qfw2434kdnl0k8fl4h3h6p55d2j6k6czz3",
  "amount": [
    {
      "unit": "lovelace",
      "quantity": "20000000"
    }
  ],
  "stake_address": "stake_test1upnhn29zpdwaje5myechf05094s4crp04vnzusqlwg59f4qjx6t73",
  "type": "shelley",
  "script": false
}

This PR:

{
  "address": "addr_vkh1vt2ntv45rx6e7rfk2qfw2434kdnl0k8fl4h3h6p55d2j6k6czz3",
  "amount": [
    {
      "unit": "lovelace",
      "quantity": "20000000"
    }
  ],
  "stake_address": null,
  "type": "shelley",
  "script": false
}