fix(notifications_app): Handle notifications without icon

[provokateurin]

I assumed there would always be an icon, but apparently it's not necessary (though almost any app does it).
Don't get confused by the nullability of the icon property, it depends on the API v2 which we always use, so we only have to check for an empty string.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.90%. Comparing base (daee3d2) to head (b389406).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2663   +/-   ##
=======================================
  Coverage   28.90%   28.90%           
=======================================
  Files         371      371           
  Lines      136616   136623    +7     
=======================================
+ Hits        39483    39491    +8     
+ Misses      97133    97132    -1     

Flag	Coverage Δ		*Carryforward flag
account_repository	98.47% <ø> (ø)
cookie_store	99.48% <ø> (ø)		Carriedforward from a511491
dashboard_app	96.05% <ø> (ø)
dynamite	31.05% <ø> (ø)		Carriedforward from a511491
dynamite_end_to_end_test	61.69% <ø> (ø)		Carriedforward from a511491
dynamite_runtime	85.40% <ø> (ø)		Carriedforward from a511491
interceptor_http_client	97.18% <ø> (ø)		Carriedforward from a511491
neon_dashboard	96.05% <ø> (ø)		Carriedforward from a511491
neon_framework	61.43% <ø> (ø)
neon_http_client	94.32% <ø> (+0.70%)	⬆️	Carriedforward from a511491
neon_notifications	100.00% <ø> (ø)		Carriedforward from a511491
neon_storage	94.66% <ø> (ø)		Carriedforward from a511491
neon_talk	99.45% <ø> (ø)		Carriedforward from a511491
nextcloud	24.33% <ø> (ø)		Carriedforward from a511491
notifications_app	97.43% <100.00%> (+0.03%)	⬆️
notifications_push_repository	98.11% <ø> (ø)
sort_box	90.90% <ø> (ø)		Carriedforward from a511491
talk_app	99.01% <ø> (+<0.01%)	⬆️

*This pull request uses carry forward flags. Click here to find out more.

Files with missing lines	Coverage Δ
...otifications_app/lib/src/widgets/notification.dart	95.74% <100.00%> (+0.09%)	⬆️

... and 1 file with indirect coverage changes

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