Fix some warnings (#452)

* Move `result` up and return it

This allows `gcc` to elide the copy in the return and keeps it from
warning with `-Wnvro` enabled.

* Remove harmful dtor definition

The definition of the dtor is not necessary and actually harmful since
the definition of a class's implicit copy constructor is deprecated if
that class has a user-declared constructor as of C++11.  Compilers can
warn about this with `-Wdeprecated`.

---------

Co-authored-by: Christoph Weiss <weiss@wsoptics.de>

include/cxxopts.hpp

@@ -1299,8 +1299,6 @@ template <>
 class standard_value<bool> : public abstract_value<bool>
 {
   public:
-  ~standard_value() override = default;
-
   standard_value()
   {
     set_default_and_implicit();
@@ -2773,18 +2771,18 @@ Options::help_one_group(const std::string& g) const
 {
   using OptionHelp = std::vector<std::pair<String, String>>;
 
+  String result;
+
   auto group = m_help.find(g);
   if (group == m_help.end())
   {
-    return "";
+    return result;
   }
 
   OptionHelp format;
 
   std::size_t longest = 0;
 
-  String result;
-
   if (!g.empty())
   {
     result += toLocalString(" " + g + " options:\n");