Check that UseSansSerif is set to something valid.

Also update CHANGES.rst for this change

Author: rocky

mathics/builtin/inout.py

@@ -2078,7 +2078,7 @@ def apply_mathml(self, expr, evaluation) -> Expression:
         query = evaluation.parse("Settings`$UseSansSerif")
         usesansserif = query.evaluate(evaluation).to_python()
         if not is_a_picture:
-            if usesansserif:
+            if isinstance(usesansserif, bool) and usesansserif:
                 xml = '<mstyle mathvariant="sans-serif">%s</mstyle>' % xml
 
         mathml = '<math display="block">%s</math>' % xml  # convert_box(boxes)