Allow to pass empty string to programPath.

[levz0r]

I ran into a problem when using pkg. As known, pkg packages all JS source files and creates an executable file. It also sets the process.execPath in run-time to the full path of the executable file. If one wants to run the said executable as an os-service - programPath should not be set (in contrast, when we want to start the service using nodejs, both nodePath and programPath must be set to work properly.
Hence the PR.