Make url optional when bot only features response

[SystemKeeper]

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Is your feature request related to a problem? Please describe.
When installing a bot that only sends message (i.e. has only feature response set), it is still required to provide a URL to occ talk:bot:install.

Describe the solution you'd like
The parameter url should only be required when the feature set includes webhook.

Describe alternatives you've considered
Provide an invalid URL for "send-only bots".

Additional context
Since it is allowed to change the feature set via occ talk:bot:state, #11357 is either required before, or enabling the webhook feature is not possible for bots created without an url.

[nickvergessen]

As per #11357 (comment) the URL is currently used as unique identifier and therefore this is not possible atm.