Fixed logic when passing empty 'visit_end_time' field to instrument server in 'setup_multigrid_watcher'

[tieneupin]

visit_end_time is either a datetime object or None. str(None) is truth-y, and will fail the validation check on the instrument server side, since "None" cannot be evaluated into a datetime object. We thus need to pass in None as-is and only stringify datetime objects.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 31.80%. Comparing base (f8a49cb) to head (2cee74e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #611   +/-   ##
=======================================
  Coverage   31.80%   31.80%           
=======================================
  Files          90       90           
  Lines       10750    10750           
  Branches     1423     1423           
=======================================
  Hits         3419     3419           
  Misses       7216     7216           
  Partials      115      115           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.