Fix plugin-update --all when there are no plugins
Fix plugin-update --all when there are no plugins

Issue: when asdf is first installed, there are no plugins, and running `asdf plugin-update --all` crashes because the plugin directory doesn't exist; similarly, the command crashes if the plugins directory exists yet doesn't contain any subdirectories.

Solution: This patch changes the directory loop from using a wildcard `plugins/*` to a more robust approach: first verify the plugins directory exists, then use the`find` command to list subdirectories and correctly handle the corner case of no subdirectories.

Future: Consider printing an error message and/or advice message. Consider automatically creating the plugins directory as needed. Consider prompting the user to install some popular plugins.
Stratus3D/asdf,asdf-vm/asdf