Current LSN : Account for NULL LSN value being returned (#2372)

```golang
// ParseLSN parses the given XXX/XXX text format LSN used by PostgreSQL.
func ParseLSN(s string) (LSN, error) {
	var upperHalf uint64
	var lowerHalf uint64
	var nparsed int
	nparsed, err := fmt.Sscanf(s, "%X/%X", &upperHalf, &lowerHalf)
	if err != nil {
		return 0, fmt.Errorf("failed to parse LSN: %w", err)
	}

	if nparsed != 2 {
		return 0, fmt.Errorf("failed to parsed LSN: %s", s)
	}

	return LSN((upperHalf << 32) + lowerHalf), nil
}
```

It was recently seen for a read replica mirror that the above
`pglogrepl` function we use to get the current LSN returned an EOF error
at the first check - indicating that the LSN string we were giving it
was not of the right format.

Upon research it seems like the functions in the query we run:
```sql
SELECT CASE WHEN pg_is_in_recovery() THEN pg_last_wal_receive_lsn() ELSE pg_current_wal_lsn() END
```
can return `NULL` as seen
[here](https://github.com/postgres/postgres/blob/1f0de66ea2a5549a3768c67434e28a136c280571/src/backend/access/transam/xlogfuncs.c#L338)
for example.

It seems that this can occur when there is no more WAL activity to
collect, not sure

This PR simply adds more logging around this.

---------

Co-authored-by: Kevin Biju <[email protected]>
Co-authored-by: Philip Dubé <[email protected]>

flow/connectors/postgres/client.go

@@ -657,9 +657,16 @@ func (c *PostgresConnector) getCurrentLSN(ctx context.Context) (pglogrepl.LSN, e
 	var result pgtype.Text
 	err := row.Scan(&result)
 	if err != nil {
-		return 0, fmt.Errorf("error while running query: %w", err)
+		return 0, fmt.Errorf("error while running query for current LSN: %w", err)
 	}
-	return pglogrepl.ParseLSN(result.String)
+	if !result.Valid || result.String == "" {
+		return 0, errors.New("error while getting current LSN: no LSN available")
+	}
+	lsn, err := pglogrepl.ParseLSN(result.String)
+	if err != nil {
+		return 0, fmt.Errorf("error while parsing LSN %s: %w", result.String, err)
+	}
+	return lsn, nil
 }
 
 func (c *PostgresConnector) getDefaultPublicationName(jobName string) string {