[Backport MB-38194 to 6.0.5] In presence of an index on empty bucket, planner can yield non-uniform partition placement.

Data size and memory usage of an index on empty bucket will be zero. Due to this planner estimates these values for this "already built" index. In some cases, this leads planner to place partitions of the new partitioned index non-uniformly.

Steps to reproduce:
1. Setup MOI cluster with 2 indexer nodes.
2. Create Empty bucket. (No mutations).
3. Create non-partitioned index. idx1.
4. Create partitioned index idx2.

In some cases, we can observe that all partitions of idx2 get placed on a single indexer node.