[Index templates] Simulate API does not return mappings when is empty

[sebelga]

Currently, when we execute the _index_template/_simulate API without providing anything, we get back empty objects for "settings" and "aliases" but not for "mappings".

It would be better and more consistent to also return an empty object for "mappings" inside the "template".

// Current response
{
  "template": {
    "settings": {},
    "aliases": {}
  },
  "overlapping": []
}

// Expected response
{
  "template": {
    "mappings": {},
    "settings": {},
    "aliases": {}
  },
  "overlapping": []
}

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Indices APIs)