fixup! Reduce number of constraints in format

Author: romainbrenguier

src/solvers/refinement/string_constraint_generator_format.cpp

@@ -397,7 +397,7 @@ exprt string_constraint_generatort::add_axioms_for_format(
 
   exprt return_code = from_integer(0, get_return_code_type());
 
-  if(intermediary_strings.size() == 0)
+  if(intermediary_strings.empty())
   {
     lemmas.push_back(equal_exprt(res.length(), from_integer(0, index_type)));
     return return_code;