Fix handling of symbols with no value

Because there was previously an assumption that all symbols in the
symbol table had values and it is more straight forward to make the this
code deal with that than to add an invariant.

Author: thomasspriggs

src/solvers/smt2_incremental/smt2_incremental_decision_procedure.cpp

@@ -67,7 +67,7 @@ void smt2_incremental_decision_proceduret::define_dependent_functions(
     {
       const irep_idt &identifier = symbol_expr->get_identifier();
       const symbolt *symbol = nullptr;
-      if(!ns.lookup(identifier, symbol))
+      if(!ns.lookup(identifier, symbol) && !symbol->value.is_nil())
       {
         if(dependencies_needed(symbol->value))
           continue;