Avoid error calculating UTR on features that have no exon subfeatures

[cmdcolin]

As title says, UTR prediction is not done if no exon subfeatures are available. Fixes #1983

[codecov]

Codecov Report

Merging #1985 (62f6568) into main (905e690) will decrease coverage by 0.03%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##             main    #1985      +/-   ##
==========================================
- Coverage   61.38%   61.35%   -0.04%     
==========================================
  Files         470      470              
  Lines       22216    22222       +6     
  Branches     5100     5100              
==========================================
- Hits        13637    13634       -3     
- Misses       8304     8313       +9     
  Partials      275      275              

Impacted Files	Coverage Δ
.../core/BaseFeatureWidget/SequenceFeatureDetails.tsx	51.51% <14.28%> (-1.95%)	⬇️
...inearGenomeView/components/RefNameAutocomplete.tsx	89.36% <0.00%> (-4.26%)	⬇️
packages/core/util/index.ts	78.19% <0.00%> (-0.27%)	⬇️

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