ui: prune: fix sending invalid parameters

ui: prune: fix sending invalid parameters

the prune input panel is used in various contexts (add/editing a
prunejob, adding a datastore, executing a prune). These different api
calls don't all take the same parameters, so we have to correctly set
the `isCreate` to not send a `delete` paramter for those request if
there was an empty field.

Also set 'max-depth:0' only when recursive was not set *and* we can
set 'recursive', because for creating a datastore that is not supported
by the api, and for the prune job editing we override the whole
onGetValues anyway so that's not an issue there.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>