Don't raise exception in evaluate() if parsing expr returns None (#1547)

This can happen for example if you give it a string consisting of just a
comment.

Author: bdlucas1

mathics/session.py

@@ -19,6 +19,7 @@
 from mathics.core.definitions import Definitions
 from mathics.core.evaluation import Evaluation, Result
 from mathics.core.parser import MathicsSingleLineFeeder, parse
+from mathics.core.symbols import SymbolNull
 
 
 def autoload_files(
@@ -150,7 +151,7 @@ def evaluate(self, str_expression, timeout=None, form=None):
         )
         if form is None:
             form = self.form
-        self.last_result = expr.evaluate(self.evaluation)
+        self.last_result = expr.evaluate(self.evaluation) if expr else SymbolNull
         return self.last_result
 
     def evaluate_as_in_cli(self, str_expression, timeout=None, form=None, src_name=""):