resolves #

Problem

When a external_volume is omitted in the configuration of an Iceberg table, the create table statement uses external_volume='None'.

Solution

Omit external_volume from relation's DDL when it's not provided.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX