Fix possible nil error on commit hash assign in case the contexts of event is nil

[khanhtc1202]

What this PR does:

as title

Why we need it:

If the event's context is nil, the trailers value will be nil, and leads to nil assignment when we try to add value to TraceTriggerCommitHashKey

Which issue(s) this PR fixes:

Follow #5625

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[ffjlabo]

LGTM

[Warashi]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.

Project coverage is 25.56%. Comparing base (bc1a61b) to head (9189946).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/app/piped/eventwatcher/eventwatcher.go	0.00%	5 Missing ⚠️
pkg/app/pipedv1/eventwatcher/eventwatcher.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5648      +/-   ##
==========================================
+ Coverage   25.54%   25.56%   +0.01%     
==========================================
  Files         477      477              
  Lines       50945    51078     +133     
==========================================
+ Hits        13016    13060      +44     
- Misses      36924    37013      +89     
  Partials     1005     1005              

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