fix(api/dashboards): return empty list

[wass3rw3rk]

when there are no dashboards we currently return 'null'. this change will return an empty list instead

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 53.05%. Comparing base (809af2a) to head (c5d500c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
api/dashboard/list_user.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1185   +/-   ##
=======================================
  Coverage   53.05%   53.05%           
=======================================
  Files         559      559           
  Lines       19740    19740           
=======================================
  Hits        10473    10473           
  Misses       8699     8699           
  Partials      568      568           

Files with missing lines	Coverage Δ
api/dashboard/list_user.go	0.00% <0.00%> (ø)