Fix check on value emptiness in polynomial acceleration

Without z3 installed the regession tests for this don't correctly
exercise this part of the code. This fix addresses a bug which appears
to have been introduced in a previous refactor to replace `nil_typet`
with `optionalt<typet>`.

Author: thomasspriggs

src/goto-instrument/accelerate/polynomial.cpp

@@ -35,7 +35,7 @@ exprt polynomialt::to_expr()
         t_it!=m_it->terms.end();
         ++t_it)
     {
-      if(itype.has_value())
+      if(!itype.has_value())
       {
         itype=t_it->var.type();
       }