Throw 'MySqlException' when field is null

[tigerlee]

It's seems this issue comes after you refactoring the lib. Former version before SHA: e0a8740c5c781203265bb9b06f9fdb635d34053a works fine with null field.

Exception:

terminate called after throwing an instance of 'MySqlException'
  what():  Null value encountered with non-smart-pointer output type

Partial stack trace:

#0  0x00007f992a8e4f77 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#1  0x00007f992a8e85e8 in __GI_abort () at abort.c:90
#2  0x00007f992b40d6e5 in __gnu_cxx::__verbose_terminate_handler() () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3  0x00007f992b40b856 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#4  0x00007f992b40b883 in std::terminate() () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#5  0x00007f992b40baae in __cxa_throw () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#6  0x0000000000410bc4 in OutputBinderPrivate::OutputBinderResultSetter<std::string>::setResult (value=0x7fff0270fea0, bind=...) at mysql-cpp/OutputBinder.hpp:335
#7  0x000000000041b57a in OutputBinderPrivate::setResultTuple<std::tuple<std::string, unsigned int, unsigned int, double, double, unsigned int, unsigned int, unsigned int, unsigned int, double, unsigned int, unsigned int, int, int, std::string>, 14> (tuple=0x7fff0270fea0, outputParameters=...) at mysql-cpp/OutputBinder.hpp:181