target overwrites state if no state message is sent

[shopmatfournier]

The target does not check whether or not the state object exists when it writes out the state here: https://github.com/z3z1ma/target-bigquery/blob/main/target_bigquery/target.py#L529 . If no state message is processed, the target will write out the empty dictionary as the updated state, overwriting any existing state.

• There is nothing in the Singer (or Meltano) spec that indicates what to do in the following situation: an INCREMENTAL tap runs but produces no records.
  • For example, maybe it runs and using the REPLICATION_KEY as part of a query there are no new records. In this instance, the Meltano SDK will not produce a STATE message if GET_RECORDS produces no values (https://github.com/meltano/sdk/blob/main/singer_sdk/streams/core.py#L1269)
  • Different taps (non-meltano) have differeing behavior. For example, tap prometheus will emit a state message even if it produces no records (https://github.com/meshcloud/tap-prometheus/blob/master/tap_prometheus/__init__.py#L204)

Suggested fix: before writing state, check if the state exists. If it does not exist, do not write an empty state message.