Stop airflowctl update commands from clearing fields left unset

[DHBhensdadia]

airflowctl connections update --connection-id pg_prod --conn-type postgres --host db2.example.com
erases that connection's login, port, schema and description.

The API applies every key present in a PATCH body, so a field the user never typed arrives as null
and is stored as one. argparse fills an omitted flag with None, and the generated command builds
the whole datamodel from those values, so by the time the body is serialized the client can no longer
distinguish "not supplied" from "set to null".

The create methods in operations.py already pass exclude_none=True; the update methods did
not. This is the same omission reported in #70327 for dags trigger, where it shows up as a request
older API servers reject rather than as lost data.

Reproduction

Against a real Airflow 3.4.0 API server on SQLite (no Docker):

$ airflowctl connections update --connection-id pg_prod --conn-type postgres --host db2.example.com

before  {"connection_id":"pg_prod","conn_type":"postgres","description":"primary warehouse",
         "host":"db1.example.com","login":"airflow","schema":"analytics","port":5432}
after   {"connection_id":"pg_prod","conn_type":"postgres","description":null,
         "host":"db2.example.com","login":null,"schema":null,"port":null}

$ airflowctl variables update --key api_endpoint --value https://staging.example.com

before  {"key":"api_endpoint","value":"https://prod.example.com","description":"prod endpoint - owned by data team"}
after   {"key":"api_endpoint","value":"https://staging.example.com","description":null}

$ airflowctl pools update --pool etl_pool --slots 42

before  {"name":"etl_pool","slots":5,"description":"ETL workers - do not delete"}
after   {"name":"etl_pool","slots":42,"description":null}

With this change, each of those commands sends only the fields the user passed, and the untouched
fields keep their stored values.

Scope

PoolsOperations.update still sends include_deferred: false, because False is not None. That
has a different cause — _get_bool_arg_default in ctl/cli_config.py returns False for every
datamodel outside its one-entry allowlist instead of the field's declared default — and is left for a
separate PR.

Also deliberately unchanged:

• DagsOperations.update — DAGPatchBody has a single required bool field, so exclude_none would
  be a no-op with nothing to test.
• The bulk methods — they PATCH a collection endpoint with a body of actions, where a null inside
  an action may be meaningful. Different semantics.
• ConnectionsOperations.test — POST, not PATCH; it does not mutate stored state.
• default_pool remains un-updatable through airflowctl (Only slots and included_deferred can be modified on Default Pool). The server only permits it when update_mask is sent, and the client
  never sends one. Separate fix.

test_update_uses_schema_alias_in_request_body asserted the old body including its null keys; its
expectation is updated. Its subject — that the schema alias is used rather than schema_ — is
unchanged and still asserted.

related: #70327

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