fix(datepicker): check trigger strategy is defined before destroying

[CandidCodingCanary]

Please read and mark the following check list before creating a pull request:

• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

Prevents the DatePickerComponent from crashing if it is destroyed shortly after its creation. View this issue for a more detailed description: #2007.

This error occurs without the null check:

[yggg]

Thanks @CandidCodingCanary! 👍

[codecov]

Codecov Report

Merging #2011 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2011      +/-   ##
==========================================
+ Coverage   84.19%   84.19%   +<.01%     
==========================================
  Files         235      235              
  Lines        7902     7903       +1     
  Branches      692      693       +1     
==========================================
+ Hits         6653     6654       +1     
  Misses       1046     1046              
  Partials      203      203

Impacted Files	Coverage Δ
...heme/components/datepicker/datepicker.component.ts	90.15% <100%> (+0.05%)	⬆️