fix(cdc-connector): allow empty passwords

[fuyufjh]

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What's changed and what's your intention?

Allow passwords to be empty for CDC connectors. Tested with my local PostgresQL instance.

Before:

dev=> CREATE SOURCE pg_mydb WITH (
    connector = 'postgres-cdc',
    hostname = '127.0.0.1',
    port = '5432',
    username = 'eric',
    password = '',
    database.name = 'test',
    slot.name = 'rw_slot'
);
ERROR:  Failed to run the query

Caused by these errors (recent errors listed first):
  1: gRPC request to meta service failed: Internal error
  2: failed to create source worker
  3: failed to create SplitEnumerator
  4: source cannot pass validation
  5: INVALID_ARGUMENT: 'password' not found, please check the WITH properties

After:

dev=> CREATE SOURCE pg_mydb WITH (
    connector = 'postgres-cdc',
    hostname = '127.0.0.1',
    port = '5432',
    username = 'eric',
    password = '',
    database.name = 'test',
    slot.name = 'rw_slot'
);
CREATE_SOURCE

dev=> CREATE TABLE t1 (
    k bigint,
    v varchar,
    PRIMARY KEY(k)
) FROM pg_mydb TABLE 'public.t1';
CREATE_TABLE

dev=> select * from t1;
 k |  v
---+-----
 1 | foo
(1 row)

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[StrikeW]

Thanks for the pr.