Fix loading external plugins in embedded linear-genome-view demo

[cmdcolin]

Since version v2.1.6 https://jbrowse.org/jb2/blog/2022/10/19/v2.1.6-release/ the 'loading external plugins in embedded lgv' demo has been broken. This is due to changes in the PluginLoader to support loading relative paths (relative to a window.location.href or baseUrl) in jbrowse-web

This came from a confusion in my case that new URL(someUrl,baseUrl='') would just ignore the baseUrl, but it actually throws an exception when baseUrl is an empty string. This updates the code to use undefined instead of empty string, and then wires the code so undefineds flow through the pluginLoader instead of empty string

[codecov]

Codecov Report

Merging #3482 (ab3e757) into main (b3f0651) will not change coverage.
The diff coverage is 33.33%.

❗ Current head ab3e757 differs from pull request most recent head 7a85bf2. Consider uploading reports for the commit 7a85bf2 to get more accurate results

@@           Coverage Diff           @@
##             main    #3482   +/-   ##
=======================================
  Coverage   61.41%   61.41%           
=======================================
  Files         820      820           
  Lines       29498    29498           
  Branches     7056     7055    -1     
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  Hits        18115    18115           
  Misses      11222    11222           
  Partials      161      161           

Impacted Files	Coverage Δ
...owse-react-circular-genome-view/src/loadPlugins.ts	0.00% <0.00%> (ø)
...browse-react-linear-genome-view/src/loadPlugins.ts	0.00% <0.00%> (ø)
packages/core/PluginLoader.ts	49.42% <50.00%> (ø)

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