Incoming Webhook must be nullable

[BenDenger]

Currently, the incomingWebhook property of our Team-object is not nullable.
That causes that the user/developer has to set the incoming_webhook for his bot, otherwise it causes an error and the installation fails.

Expected Behavior

The incoming webhook property should be nullable, so the incoming_webhook is not necessary.

Current Behavior

it is necessary

Possible Solution

make the property nullable

Steps to Reproduce

install the bot, without setting the incoming webhook

[BenDenger]

For some reason its not nullable anymore