[FIX] Error when mentioning a non-member of a public channel

[debdutdeb]

Proposed changes (including videos or screenshots)

Fixes the error:

errorClass [Error]: [TypeError: subscriptions.find is not a function]
    at app/lib/server/lib/sendNotificationsOnMessage.js:392:11
    at /app/bundle/programs/server/npm/node_modules/meteor/promise/node_modules/meteor-promise/fiber_pool.js:43:40 {
  isClientSafe: true,
  error: TypeError: subscriptions.find is not a function
      at app/lib/server/lib/sendNotificationsOnMessage.js:377:41
      at Array.forEach (<anonymous>)
      at app/lib/server/lib/sendNotificationsOnMessage.js:376:11
      at /app/bundle/programs/server/npm/node_modules/meteor/promise/node_modules/meteor-promise/fiber_pool.js:43:40,
  reason: undefined,
  details: undefined,
  errorType: 'Meteor.Error'
}

Issue(s)

Fixes #26908

Steps to test or reproduce

Further comments

[codecov]

Codecov Report

Merging #26917 (d7cf19a) into develop (5c26dd9) will increase coverage by 0.02%.
The diff coverage is n/a.

❗ Current head d7cf19a differs from pull request most recent head d2fb9df. Consider uploading reports for the commit d2fb9df to get more accurate results

@@             Coverage Diff             @@
##           develop   #26917      +/-   ##
===========================================
+ Coverage    40.89%   40.91%   +0.02%     
===========================================
  Files          791      791              
  Lines        17809    17809              
  Branches      1928     1928              
===========================================
+ Hits          7283     7287       +4     
+ Misses       10233    10228       -5     
- Partials       293      294       +1     

Flag	Coverage Δ
e2e	40.91% <ø> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.