Fix: corrected empty double quotes being added to cmd.exe /c on Windo…

[fkonvick]

We used to run bp::child(bp::shell, bp::args = {shell_script}, ... which resulted in C:\Windows\system32\cmd.exe /c shell_script out of build_args.

After #256 we're getting C:\Windows\system32\cmd.exe /c "" shell_script (note the extra empty pair of double quotes) and the script refuses to run.

This code in exe_args_shell with Process v1 on Windows:

std::vector<string_type> args_ = {c_arg(Char()), std::move(exe)};
args_.insert(args_.end(), std::make_move_iterator(args.begin()), std::make_move_iterator(args.end()));

adds exe to args_, but this might be empty, especially with bp::shell, and (newly after #256) gets translated to unwanted "" rather than being omitted, which causes prior user code to break.

This PR only adds exe to args_ if non-empty, so it fixes the use case described above.

Also see my comments in #256 and related #73 for further info.

Using bp::exe = {shell_script} instead of bp::args might be a workaround for some.