Fix bug where multi-quantitative tracks plotted incorrectly for some bigwigs with empty regions

[cmdcolin]

Fixes #3838

The key is that I assumed that all subtracks would return some feature for splitting into groups and getting a reliable number of "groups" but instead we can use the "sources" variable that is passed as a prop

(Sources is something that the display passes to the renderer, and the display itself requests the "sources" of the multiquantitative track as a separate RPC call. just for interest, the number of subtracks could change regionally in this way)

[codecov]

Codecov Report

Merging #3839 (168cc7e) into main (1c43657) will increase coverage by 0.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main    #3839      +/-   ##
==========================================
+ Coverage   64.39%   64.40%   +0.01%     
==========================================
  Files         999      999              
  Lines       29710    29710              
  Branches     7125     7125              
==========================================
+ Hits        19133    19136       +3     
+ Misses      10415    10412       -3     
  Partials      162      162              

Files Changed	Coverage Δ
...e/src/MultiDensityRenderer/MultiDensityRenderer.ts	0.00% <0.00%> (ø)
...e/src/MultiRowLineRenderer/MultiRowLineRenderer.ts	0.00% <0.00%> (ø)
...c/MultiRowXYPlotRenderer/MultiRowXYPlotRenderer.ts	95.83% <100.00%> (ø)

... and 1 file with indirect coverage changes

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