build(python): make `NOX_SESSION` optional (#135)

I added this accidentally in #889. `NOX_SESSION` should be passed down if it is set but not marked required.

Source-Author: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>
Source-Date: Tue Jan 19 09:38:04 2021 -0700
Source-Repo: googleapis/synthtool
Source-Sha: ba960d730416fe05c50547e975ce79fcee52c671
Source-Link: https://github.com/googleapis/synthtool/commit/ba960d730416fe05c50547e975ce79fcee52c671
2 files changed