Handle empty result from oldest results query.

As clients can have no results in normal circumstances.
Let guard commands run in dry run

To make dry-run meaningful for configuration commands
Merge pull request #15 from jaeheung90/debug_dry-run_guard

Let guard exec run in dry-run
Merge pull request #14 from jaeheung90/debug_app_views_clients

Bug fix: Handle empty result from oldest results query.