Added check for blank on name entry

Resolves issue #5747
Since cancel just returns a blank this checks for that and doesn't raise an error.
I think it would be simpler to just do:
```
var/new_name = reject_bad_name( input(user, "Choose your character's name:", "Character Preference") as text|null )
if(new_name)
    real_name = new_name
else if (!isnull(new_name))
    user << "<font color='red'>Invalid name. Your name should be at least 2 and at most [MAX_NAME_LEN] characters long. It may only contain the characters A-Z, a-z, -, ' and .</font>"
```