Remove pass keyword

Pass keyword is not brought up in tutorial so far and would therefor be confusing to new students, replaced with empty returns.

Author: s0rta

tutorials/learnpython.org/en/Functions.md

@@ -81,11 +81,11 @@ Tutorial Code
 
 # Modify this function to return a list of strings as defined above
 def list_benefits():
-    pass
+    return []
 
 # Modify this function to concatenate to each benefit - " is a benefit of functions!"
 def build_sentence(benefit):
-    pass
+    return ""
 
 def name_the_benefits_of_functions():
     list_of_benefits = list_benefits()