Empty strings are "falsy" which means they are considered false in a Boolean context, so you can just use not string.
string = "" if not string: print "Empty String!"
This will give the output:
Empty String!
If your string can have whitespace and you still want it to evaluate to false, you can just strip it and check again. For example:
string = " " if not string.strip(): print "Empty String!"
This will give the output:
Empty String!