Don't preserve empty <tip>s

problem_builder/v1/tests/xml/v1_upgrade_b_new.xml

@@ -26,7 +26,6 @@
   </pb-mcq>
   <pb-rating name="frog-important" low="Not at all important to me" high="Very important to me" question="How important is it to you?" correct_choices='["4","5","1","2","3","understand"]'>
     <pb-choice value="understand">I don't understand</pb-choice>
-    <pb-tip values='["4","5"]'>Great!</pb-tip>
     <pb-tip values='["1","2","3"]'>The Trial of Uruk-Shan helps you uncover some of the core beliefs and assumptions you have held that are preventing you from making change.</pb-tip>
     <pb-tip values='["understand"]'>A frog is important if it is one that could make a big difference in helping you reach your frogs in your work life or your personal life (or both).</pb-tip>
   </pb-rating>

problem_builder/v1/tests/xml/v1_upgrade_b_old.xml

@@ -40,7 +40,7 @@ This contains a typical problem taken from a live course (content changed)
           <quizz name="frog-important" type="rating" low="Not at all important to me" high="Very important to me">
             <question>How important is it to you?</question>
             <choice value="understand">I don't understand</choice>
-            <tip display="4,5">Great!</tip>
+            <tip display="4,5"> </tip>
             <tip display="1,2,3">The Trial of Uruk-Shan helps you uncover some of the core beliefs and assumptions you have held that are preventing you from making change.</tip>
             <tip display="understand">A frog is important if it is one that could make a big difference in helping you reach your frogs in your work life or your personal life (or both).</tip>
           </quizz>

problem_builder/v1/xml_changes.py

@@ -283,6 +283,9 @@ class TipChanges(Change):
                 warnings.warn("Invalid <tip> element found.")
                 return
         self.node.attrib["values"] = value
+        if (self.node.text is None or self.node.text.strip() == "") and not list(self.node):
+            # This tip is blank.
+            p.remove(self.node)
 
 
 class AlternatingHTMLToQuestions(Change):