FieldValueFactorFunction should use 'missing' when field does't exist in mapping

[edudar]

It's common case for continuous deployment to change elasticsearch's client and index independently. This leads to the case when fieldValueFactorFunction is added to the client but index is not rebuilt yet. I use missing property. Unfortunately FieldValueFactorFunctionParser demands to have a field in a mapping, it throws exception otherwise. This means we can't deploy an client before index is updated despite having missing property set. The suggestion here is to look as missing in case of field in the document or mapping are missing.

[clintongormley]

Duplicate of #12016