fix(date service): prevent format calls if date isn't passed

[yggg]

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Short description of what this resolves:

Fixes #1251

[codecov]

Codecov Report

Merging #1291 into master will increase coverage by 0.4%.
The diff coverage is 90.9%.

@@           Coverage Diff            @@
##           master   #1291     +/-   ##
========================================
+ Coverage   79.49%   79.9%   +0.4%     
========================================
  Files         232     232             
  Lines        7024    7032      +8     
  Branches      587     590      +3     
========================================
+ Hits         5584    5619     +35     
+ Misses       1272    1244     -28     
- Partials      168     169      +1

Impacted Files	Coverage Δ
.../theme/components/datepicker/datepicker-adapter.ts	100% <100%> (+35.29%)	⬆️
...amework/date-fns/services/date-fns-date.service.ts	76.19% <100%> (+2.5%)	⬆️
...c/framework/moment/services/moment-date.service.ts	93.15% <66.66%> (+0.19%)	⬆️
...heme/components/datepicker/datepicker.component.ts	83.92% <0%> (+2.38%)	⬆️
...heme/components/datepicker/datepicker.directive.ts	79.5% <0%> (+9.01%)	⬆️

[nnixaa]

@yggg can we add a test for datepicker-adapter.ts?

[yggg]

@yggg can we add a test for datepicker-adapter.ts?

@nnixaa Done in 7e1cada and 953ffc9.

Please note, I also slightly changed the logic of the format method of NbRangeAdapterService (8b7cc71). Now it tries to format end date only if the start date is valid.