After recent change (1a32ccd) `make_update_indices_mutations()` is unconditionally adding a mutation for `system.view_building_tasks`, even when no indices were being dropped.
In a mixed-version cluster, the older node may not have this table, causing the Raft schema applier to fail with 'Can't find a column family with UUID ...'.
This patch fixes the bug by emitting the mutation when indices are actually dropped (i.e., when the view building cleanup code path was entered).
Fixes: SCYLLADB-2026
Refs: scylladb#26557
scylladb#26557 wasn't backported, so this patch also doesn't need to be.
Closesscylladb/scylladb#29908
* github.com:scylladb/scylladb:
db/schema_tables: don't emit empty view_building_tasks mutation on ALTER TABLE
db/view_building_task_mutation_builder: add `empty()` method
5efb43195e