You could just compare your string to the empty string:
if variable != "":
etc.
But you can abbreviate that as follows:
if variable:
etc.
Explanation: An if actually works by computing a value for the logical expression you give it: True or False. If you simply use a variable name (or a literal string like “hello”) instead of a logical test, the rule is: An empty string counts as False, all other strings count as True. Empty lists and the number zero also count as false, and most other things count as true.