`cmd.New(programWithSpaces)` will now work for executables that have
spaces in the path. The only place where we relied on shell-splitting
was in openTextEditor, which now performs its own splitting.

Empty string arguments are removed when building an executable command, causing problems like https://github.com/github/hub/issues/725. However, we need to remove empty string arguments constructed from the commands layer, e.g., https://github.com/github/hub/blob/master/commands/browse.go#L112. We fix it by moving the removal logic to `Args.ToCmd`.

This reverts commit 65cd2b86aead127224f1911cd6e73aa92f75320a.