MongoDB\Driver\WriteResult getters currently return null if the libmongoc result document omits an expected field.

• Continue to return null for unacknowledged writes, but raise a deprecation notice informing users that this may change in 2.0 (see: PHPC-2144)
• Return 0 for any acknowledged writes if the field is not present. This may not actually happen in libmongoc, but it'd be a good defensive approach since results are BSON documents.