Fix potential crashes where alignment SEQ field may be empty, simplify some display model code

[cmdcolin]

SEQ is often left empty on secondary alignments, and this causes at least one error in our code (per-base coloring on alignments tracks)

This adds typescript notes that feature.get('SEQ') returns string|undefined and fixes related issues

[codecov]

Codecov Report

Merging #3620 (de6571f) into main (9fe2420) will decrease coverage by 0.02%.
The diff coverage is 19.23%.

@@            Coverage Diff             @@
##             main    #3620      +/-   ##
==========================================
- Coverage   62.96%   62.94%   -0.02%     
==========================================
  Files         871      871              
  Lines       30193    30199       +6     
  Branches     7271     7271              
==========================================
- Hits        19012    19010       -2     
- Misses      10996    11003       +7     
- Partials      185      186       +1     

Impacted Files	Coverage Δ
packages/core/ReExports/modules.tsx	11.16% <ø> (ø)
...rc/LinearPileupDisplay/components/SetMaxHeight.tsx	0.00% <0.00%> (ø)
...nts/src/SNPCoverageAdapter/generateCoverageBins.ts	50.84% <0.00%> (-0.88%)	⬇️
...ns/alignments/src/PileupRenderer/PileupRenderer.ts	51.80% <18.18%> (-0.64%)	⬇️
...lugins/alignments/src/LinearPileupDisplay/model.ts	66.25% <50.00%> (-0.14%)	⬇️
...ative-view/src/LinearReadVsRef/LinearReadVsRef.tsx	74.22% <100.00%> (ø)
...ns/wiggle/src/LinearWiggleDisplay/models/model.tsx	72.07% <100.00%> (-0.38%)	⬇️

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