Always append shard path in diags

[otoolep]

This code is clearer -- simply append an empty path if the shard is not
local.

Fixes issue #2430

[otoolep]

Before this change the column path was added to the diags columns if the shard was local, otherwise it was not. That code had some bugs, so it's much simpler to just add the path column for all shards and simply set the path value to "" for non-local shards.

[marcosnils]

@otoolep PR is clean and better. 👍 . I didn't want to change too much as I still don't know how internals work.

[otoolep]

@mark-rushakoff

[toddboom]

+1