python: fix segfault on empty argv in main()

[senyai]

test:
python -c 'import google_benchmark as b;b.register(print);b.main([]);'

[senyai]

I think there's more explicit and expressive way to update the logic:

if (!argv.empty()) {
  static std::string executable_name(argv[0]);
  ptrs[0] = const_cast<char*>(executable_name.c_str());
}

should I change it, @LebedevRI?