Don't manipulate input params in email notification

[jpraet]

Fixes #15993

In some email notifications the content of the input model object is set to empty string "".
But this affects the other notification services, for example causing the content to also be empty in the webhook payloads.

Instead of directly manipulating the input object, the empty content is now passed mailCommentContext.

[codecov-commenter]

Codecov Report

Merging #16011 (f2ff3fd) into main (a9daebc) will decrease coverage by 0.01%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main   #16011      +/-   ##
==========================================
- Coverage   44.07%   44.06%   -0.02%     
==========================================
  Files         682      682              
  Lines       82389    82394       +5     
==========================================
- Hits        36312    36303       -9     
- Misses      40177    40190      +13     
- Partials     5900     5901       +1     

Impacted Files	Coverage Δ
modules/notification/mail/mail.go	37.89% <ø> (-0.89%)	⬇️
services/mailer/mail_comment.go	77.77% <50.00%> (-5.56%)	⬇️
services/mailer/mail_issue.go	60.49% <100.00%> (+2.59%)	⬆️
modules/util/timer.go	42.85% <0.00%> (-42.86%)	⬇️
services/pull/temp_repo.go	26.59% <0.00%> (-3.20%)	⬇️
models/unit.go	46.57% <0.00%> (-2.74%)	⬇️
services/pull/check.go	26.02% <0.00%> (-2.74%)	⬇️
services/pull/patch.go	54.23% <0.00%> (-1.70%)	⬇️
modules/queue/unique_queue_disk_channel.go	48.63% <0.00%> (+1.36%)	⬆️

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