fix(desc): empty slot not showing empty state

[kiaking]

I found that currently, if we pass null value to <SDesc> slots, it will not render <SDEscEmpty>. This PR fixes it by checking in the actual slot contents.

I've also added helper utilities to do this as well.

<!-- This works. <SDescEmpty> renderd -->
<SDescText></SDescText>

<!-- This does not work -->
<SDescText>{{ undefined }}</SDescText>

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04% 🎉

Comparison is base (94db5b7) 84.47% compared to head (d798e5e) 84.51%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #328      +/-   ##
==========================================
+ Coverage   84.47%   84.51%   +0.04%     
==========================================
  Files         139      139              
  Lines       11127    11156      +29     
  Branches      492      630     +138     
==========================================
+ Hits         9400     9429      +29     
  Misses       1727     1727              

Files Changed	Coverage Δ
lib/components/SDescDay.vue	100.00% <100.00%> (ø)
lib/components/SDescLink.vue	100.00% <100.00%> (ø)
lib/components/SDescNumber.vue	100.00% <100.00%> (ø)
lib/components/SDescText.vue	100.00% <100.00%> (ø)
lib/composables/Utils.ts	100.00% <100.00%> (ø)

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[brc-dd]

Looking at the tests, this will also treat <SDescText><div></div></SDescText> as empty, right?

[kiaking]

@brc-dd Haven't tested but I think so...? At the moment, either way works in that case since we don't have that use case 😅