Replacing materialized view when the materialized view doesnt exist causes `agate.mapped_sequence.MappedSequence object has no element 0`

[the4thamigo-uk]

Describe the bug

Steps to reproduce

1. Build a materialized view model
2. Drop the materialized view objects on the database (keep the destination table though)
3. Rebuild the materialized view model with full refresh

Code fails here because there are no columns in the returned object

agate.mapped_sequence.MappedSequence object has no element 0

(https://github.com/ClickHouse/ClickHouse/blob/master/programs/obfuscator/Obfuscator.cpp#L42-L80) if necessary

[BentsiLeviav]

Merged a fix on #396