Check for null values when processing service map

[wrijeff]

Related to opendistro-for-elasticsearch/data-prepper#479.

There is an issue that causes the list on the Services page to disappear if a record in the underlying service-map index is missing. We still don't know the exact cause on the ingestion side, but please add null checks to handle this case if part of a destination-target pair of records is missing.

Will attempt to increase resiliency in Data Prepper either through retries or by periodically re-sending service map records.

[joshuali925]

Fix is merged to ignore partial records, closing issue