Fix segfault with undefined relevant_snapshot in Iceberg metadata

[ianton-ru]

Changelog category (leave one):

• Critical Bug Fix (crash, data loss, RBAC) or LOGICAL_ERROR

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Solved #1210
Fix segfault in requests to system,table when table in Iceberg catalog does not have snapshot.

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Regression jobs to run:

• Fast suites (mostly <1h)
• Aggregate Functions (2h)
• Alter (1.5h)
• Benchmark (30m)
• ClickHouse Keeper (1h)
• Iceberg (2h)
• LDAP (1h)
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[github-actions]

Workflow [PR], commit [4619924]

[ianton-ru]

Failed test - 03644_object_storage_correlated_subqueries unrelated (fix in #1203)

[alsugiliazova]

Was not able to reproduce this issue, but found that partition_key and sorting_key are not exposed when using glue: #1235

I checked the code, relevant_snapshot variable is used safely now everywhere.

In case of rest, for the reproduction I tried to remove current_snapshot_id field from metadata.json or set it to 1.
In this case SHOW TABLES just returned 0 rows.

In case of glue, SHOW TABLES does not go to object storage, but uses glue metadata. I tried to modify some fields there (Parameters: metadata_location), or just corrupt it, but apparently SHOW TABLES only need table name and namespace to work correctly.