don't set test_args if empty

[IanButterworth]

Otherwise you get the empty kwarg entry test_args=String[]

│
│ To reproduce this CI run locally run the following from the same repository state on julia version 1.10.5:
│
│ `import Pkg; Pkg.test(;coverage=true, julia_args=["--check-bounds=yes", "--compiled-modules=yes"], test_args=String[], force_latest_compatible_version=false, allow_reresolve=true)`
│

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (f5f652a) to head (c7f601d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #127   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines            2         2           
=========================================
  Hits             2         2           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.