Feature Request: Make client in projects optional

What version of Invoice Ninja are you running? ie v4.5 / v5
v5

What environment are you running?
Shared Hosting

Have you searched existing issues/requests?
yes

Screenshots
![create-project](


Additional context
Sometimes we have projects with multiple clients included. This projects can not be represented in InvoiceNinja because you have to select exactly one client for a project.
It would help to make the client field in the projects optionally so that we don’t have to fill out this field at all.

For another company (trucking) we would like to use the projects as trucks so that we can track expenses and revenues of the trucks/ projects and to get a report for that truck/ project. Same problem there: One truck is used for different clients but currently we have to choose exactly one client for a project/ truck.