serial/tree_fs_output: don't make empty dirs for empty nodes
serial/tree_fs_output: don't make empty dirs for empty nodes

1. Delay node dir enter until first link i serialized.

2. Node dir name = node's group ID (UUID) instead of OID (can duplicate,
contain incorrect symbols, etc).

3. Throw errors and stop saving early.

uentity/blue-sky-re,uentity/blue-sky-re,uentity/blue-sky-re