Do not implicitly call std::string(0 /* nullptr */). Fixes CI build.

[inkdot7]

The CI build failures looked like this:

c++ -c -Wall -Wextra -pthread -std=c++23 -m64 -fsized-deallocation -I/opt/root/include -g -fPIC -O3 -DUNIX -DPOSIX -DLINUX -DAME_FILE=\"/__w/ISSSort/ISSSort/data/mass_1.mas20\" -DSRIM_DIR=\"/__w/ISSSort/ISSSort/srim/\" -DCUR_DIR=\"/__w/ISSSort/ISSSort/\" -I./include -I. -c src/Calibration.cc -o src/Calibration.o
src/Calibration.cc: In member function ‘std::string ISSCalibration::CaenType(unsigned int, unsigned int)’:
src/Calibration.cc:627:16: error: use of deleted function ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::nullptr_t) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::nullptr_t = std::nullptr_t]’
  627 |         return 0;
      |                ^
In file included from /usr/include/c++/15/string:56,
                 from /usr/include/c++/15/bits/locale_classes.h:42,
                 from /usr/include/c++/15/bits/ios_base.h:43,
                 from /usr/include/c++/15/ios:46,
                 from /usr/include/c++/15/bits/ostream.h:43,
                 from /usr/include/c++/15/ostream:42,
                 from /usr/include/c++/15/iostream:43,
                 from ./include/Calibration.hh:4,
                 from src/Calibration.cc:1:
/usr/include/c++/15/bits/basic_string.h:835:7: note: declared here
  835 |       basic_string(nullptr_t) = delete;
      |       ^~~~~~~~~~~~
src/Calibration.cc:627:16: note: use ‘-fdiagnostics-all-candidates’ to display considered candidates
  627 |         return 0;
      |                ^

Example: https://github.com/inkdot7/ISSSort/actions/runs/19911360575/job/57080553420

I think this suggests std::string(0) is undefined behaviour (section 'Exception safety'):

https://cplusplus.com/reference/string/string/string/

[lpgaff]

overtaken by changes in #139