Fix NPE for /_cat/indices when no primary shard (elastic#26953)

When a node which contains the primary shard is unavailable, the primary
stats (and the total stats) of an `IndexStats` will be empty for a short
moment (while the primary shard is being relocated). However, we assume
that these stats are always non-empty when handling `_cat/indices` in
RestIndicesAction. This commit checks the content of these stats before
accessing.

Closes elastic#26942

Author: dnhatn