users: Change type of enable_marketing_emails parameter in create_user.
users: Change type of enable_marketing_emails parameter in create_user.

This commit changes the type of enable_marketing_emails parameter of
create_user to Optional[bool].
The value of this parameter will be None in certain cases when user
registers through SSO and 'TERMS_OF_SERVICE=False' when there will
be no registration form and thus no value of enable_marketing_emails.

andersk/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,andersk/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,kou/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,rht/zulip,zulip/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,rht/zulip