fix bug when virutalenv_prompt_info and pyenv_prompt_info is both not null

Hi @tonyseek ,
I have found that if I am using system version in `pyenv`, it can't show me the name of virutalenv in shell.
That is cause `[ -z "$(pyenv_prompt_info)" ]` always return `false`.
And I have fixed this bug, pls review, thanks.