Convert results just if the condition is valid in If.to_sympy (#1537)

Just a small detail about converting conditional expressions to sympy.

---------

Co-authored-by: mmatera <[email protected]>

Author: mmatera

mathics/builtin/procedural.py

@@ -453,8 +453,6 @@ def eval_with_false_and_other(self, condition, t, f, u, evaluation):
     def to_sympy(self, expr, **kwargs):
         if len(expr.elements) == 3:
             sympy_cond = expr.elements[0].to_sympy(**kwargs)
-            sympy_true = expr.elements[1].to_sympy(**kwargs)
-            sympy_false = expr.elements[2].to_sympy(**kwargs)
             if sympy_cond is None:
                 return
             # sympy.Piecewise is is picky as to what type of conds it will accept,
@@ -463,12 +461,16 @@ def to_sympy(self, expr, **kwargs):
             # Seems an attempt at early typechecking, but maybe too restrictive -
             # what about Exprs in general that might or might not evaluate to Boolean?
             # See similar code in mathics.builtin.arithmetic.ConditionalExpression.
-            if (
+            if not (
                 sympy_cond.is_Boolean
                 or sympy_cond.is_Relational
                 or sympy_cond.is_Symbol
             ):
-                return sympy.Piecewise((sympy_true, sympy_cond), (sympy_false, True))
+                return
+            sympy_true = expr.elements[1].to_sympy(**kwargs)
+            sympy_false = expr.elements[2].to_sympy(**kwargs)
+
+            return sympy.Piecewise((sympy_true, sympy_cond), (sympy_false, True))
 
 
 class Interrupt(Builtin):