Fix Undefined jinja variable error with Salt 0.17.1

Ran into an error with Salt version 0.17.1:
Rendering SLS users failed, render error: Undefined jinja variable; line 32

After finding https://github.com/saltstack/salt/issues/8245, and working on
line 32, I surmised that line 28 might be the actual cause of the issue.

I just added a quick check to see if uid exists in user, just as the user uid
line does.