Allow zero "Max number of new conversations allowed"

[marc-farre]

What steps will reproduce the problem?

Create a new account and try sending a new message.

What is the expected result?

You are not allowed to send a message.

What do you get instead?

You can send a message.

Additional info

This is because of Config::canCreateConversation() which test if $maxConversations is empty:
https://github.com/humhub/mail/blob/master/models/Config.php#L115

We should distinguish 0 from ''.