Don't fail sync if a presence event contains an empty presence field

[morguldir]

Observed by using FluffyChat and conduit/conduwuit as the server

Example:

@nomad:daedric.net: {type: m.presence, content: {currently_active: false, last_active_ago: 1712913753736, presence: , status_msg...}

[CLAassistant]

All committers have signed the CLA.

[krille-chan]

Looks good to me. Thank you so much :) to be able to merge this, we would need to sign the commit with one of our company-internal keys. Can we get write access to your branch?

[morguldir]

Allow edits by maintainers was checked already, but invited you as collaborators just in case