[tests] fix the telemetry test for verifying the RLOC16

[superwhd]

Router ID is in range [0, MAX_ROUTER_ID] therefore zero is a valid RLOC16. The test case was wrongly checking if the RLOC16 is greater than zero.

An example failure: https://github.com/openthread/ot-br-posix/actions/runs/9361612569/job/25768930225?pr=2312.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 40.28%. Comparing base (2b41187) to head (23f0b15).
Report is 673 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2313       +/-   ##
===========================================
- Coverage   55.77%   40.28%   -15.49%     
===========================================
  Files          87       87               
  Lines        6890     9241     +2351     
  Branches        0      653      +653     
===========================================
- Hits         3843     3723      -120     
- Misses       3047     5340     +2293     
- Partials        0      178      +178     

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