Make sure the shards table has its index created before populating it

Signed-off-by: Rémy Coutable <remy@rymai.me>

config/initializers/fill_shards.rb

 # The `table_exists?` check is needed because during our migration rollback testing,
 # `Shard.connected?` could be cached and return true even though the table doesn't exist
-if Shard.connected? && Shard.table_exists? && !Gitlab::Database.read_only?
-  Shard.populate!
-end
+return unless Shard.connected?
+return unless Shard.table_exists?
+return unless Shard.connection.index_exists?(:shards, :name, unique: true)
+return if Gitlab::Database.read_only?
+
+Shard.populate!