fix panic when tenant does not exist

[localvar]

No description provided.

[megaeasex]

[TASK:easegress-pr-test SUCCESS]megaease/easegress Pull Request 323 Deploy Test Success

[codecov-commenter]

Codecov Report

Merging #323 (9ff49aa) into main (c050dbb) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #323      +/-   ##
==========================================
- Coverage   80.23%   80.20%   -0.04%     
==========================================
  Files          53       53              
  Lines        6098     6098              
==========================================
- Hits         4893     4891       -2     
- Misses        943      945       +2     
  Partials      262      262              

Impacted Files	Coverage Δ
pkg/object/mqttproxy/client.go	79.87% <0.00%> (-1.22%)	⬇️

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