fix: should not notify backgroundTaskHelper if teggContext not exists

[killagu]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

Description of change

[codecov]

Codecov Report

Base: 92.77% // Head: 92.75% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (7a1a108) compared to base (a33849d).
Patch coverage: 75.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
- Coverage   92.77%   92.75%   -0.02%     
==========================================
  Files         214      214              
  Lines        4509     4512       +3     
  Branches      459      460       +1     
==========================================
+ Hits         4183     4185       +2     
- Misses        326      327       +1     

Impacted Files	Coverage Δ
plugin/tegg/app.ts	97.72% <75.00%> (-2.28%)	⬇️

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