Use the toBeNull() matcher consistently in the unit/integration tests

[Snuffleupagus]

This replaces all toEqual(null) and toBe(null) occurrences.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.84%. Comparing base (05e100c) to head (60340d9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21632   +/-   ##
=======================================
  Coverage   89.84%   89.84%           
=======================================
  Files         263      263           
  Lines       66811    66811           
=======================================
  Hits        60029    60029           
  Misses       6782     6782           

Flag	Coverage Δ
integrationtest	69.26% <ø> (ø)
unittest	57.51% <ø> (+<0.01%)	⬆️
unittestcli	56.50% <ø> (ø)

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[calixteman]

LGTM. Thank you.