fix(categories): fall back to empty array

[st3iny]

Followup to #5161

The categories are not injected on some views, e.g. the public view.

Repro

1. Create a calendar and share it publicly via a link.
2. Open the calendar in a private/anonymous tab.
3. Observe how shit breaks because the initial state for categories is missing and no fallback is specified.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (9f53311) 22.71% compared to head (32fa3fa) 22.71%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #5382   +/-   ##
=========================================
  Coverage     22.71%   22.71%           
  Complexity      372      372           
=========================================
  Files           237      237           
  Lines         11679    11679           
  Branches       2270     2270           
=========================================
  Hits           2653     2653           
  Misses         9026     9026           

Flag	Coverage Δ
javascript	13.94% <0.00%> (ø)
php	65.42% <ø> (ø)

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Files Changed	Coverage Δ
src/mixins/EditorMixin.js	3.61% <0.00%> (ø)

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