zpm: only deploy ui when ui section in zapp.yaml

If the `ui` section was not present in the zapp.yaml file (default),
`zpm deploy` would try nonetheless to deploy ui files, resulting in a
crash.

This change fixes that.
This new `if` looks unnecessary since `_find_ui_uploads` now always do the right thing (including returning no paths when `'ui' not in zapp_config`).