Allow sorting on nested sub generated field (PR for 6150)

When you have a nested document and want to sort on its fields, it's perfectly doable on regular fields but not on "generated" sub fields.

Here is a SENSE recreation:

```
DELETE /tmp

PUT /tmp

PUT /tmp/doc/_mapping
{
  "properties": {
    "flat": {
      "type": "string",
      "index": "analyzed",
      "fields": {
        "sub": {
          "type": "string",
          "index": "not_analyzed"
        }
      }
    },
    "nested": {
      "type": "nested",
      "properties": {
        "foo": {
          "type": "string",
          "index": "analyzed",
          "fields": {
            "sub": {
              "type": "string",
              "index": "not_analyzed"
            }
          }
        }
      }
    }
  }
}

PUT /tmp/doc/1
{
  "flat":"bar",
  "nested":{
    "foo":"bar"
  }
}
```

When sorting on `flat.sub` sub field, everything is fine:

```
GET /tmp/doc/_search
{
  "sort": [
    {
      "flat.sub": {
        "order": "desc"
      }
    }
  ]
}

# gives
# "sort": [
#               "bar"
#            ]
```

When sorting on `nested` field, everything is fine:

```
GET /tmp/doc/_search
{
  "sort": [
    {
      "nested.foo": {
        "order": "desc"
      }
    }
  ]
}

# gives
# "sort": [
#               "bar"
#            ]
```

But when sorting on `nested.sub` field, sorting is incorrect:

```
GET /tmp/doc/_search
{
  "sort": [
    {
      "nested.foo.sub": {
        "order": "desc"
      }
    }
  ]
}

# gives
# "sort": [
#               null
#            ]
```

Closes #6150.