Postgres table in which data exists can't be upsert unless "key" is specified in the config when creating

Postgres table in which data exists can't be upsert unless "key" is specified in the config when creating  Postgres Destination connector (request: /v1/connectors). Even if the "key" field is present in the metadata
Steps to reproduce

1. There is a table with data with the  primary "key" in Postgres
2. Create a new pipeline
3. Create Postgres destination connector with configs
"settings": {
"url": "postgresql://postgres:postgres@localhost:5432/postgres",
table: "",
"key": ""
}
4. Create Source connector that transfers the "key" in metadata (For example, Postgres or Snowflake Source)
5. Run the pipeline -> the system is trying to transfer data by "key" which already exists
Actual result: data isn't transferred to the Postgres table
Expected result: The data should be updated by "key" from the metadata.

2022-05-12T13:56:39+00:00 ERR node stopped error="node 9873f4b8-61d7-483e-8b24-a52f12395ad0-acker stopped with error: error receiving ack: ERROR: duplicate key value violates unique constraint \"table2_pkey\" (SQLSTATE 23505)" component=pipeline.Service node_id=9873f4b8-61d7-483e-8b24-a52f12395ad0-acker stack=[{"file":"/Users/serhii/conduit/pkg/pipeline/lifecycle.go","func":"[github.com/conduitio/conduit/pkg/pipeline.(*Service](http://github.com/conduitio/conduit/pkg/pipeline.(*Service)).runPipeline.func1","line":387},{"file":"/Users/serhii/conduit/pkg/connector/destination.go","func":"[github.com/conduitio/conduit/pkg/connector.(*destination](http://github.com/conduitio/conduit/pkg/connector.(*destination)).Ack","line":194},{"file":"/Users/serhii/conduit/pkg/plugin/builtin/v1/destination.go","func":"[github.com/conduitio/conduit/pkg/plugin/builtin/v1.(*destinationPluginAdapter](http://github.com/conduitio/conduit/pkg/plugin/builtin/v1.(*destinationPluginAdapter)).Ack","line":137}]

Reproduce the existing-row upsert failure using the PostgreSQL destination request with an empty key and metadata containing the key. Start with the error path mentioned in pkg/pipeline/lifecycle.go, pkg/connector/destination.go, and pkg/plugin/builtin/v1/destination.go; done means the existing row is updated by the metadata key without a duplicate-key error.

Written by the indexing model from the issue text.