Fix crash when chromSizesLocation not specified when loading TwoBitAdapter in GUI

[cmdcolin]

Currently loading a twobit file from the desktop start screen fails, and fails with a big snapshot failure that crashes the app

There is a assumption in some usages of FileSelector where a "blank location object" is {uri:''}

Then, with TwoBitAdapter in particular, the "chromSizesLocation" is an optional entry that speeds up loading the twoBit, but if chromSizesLocation is not specified, then loading fails.

Specifically, it fails to go through the FileLocation preprocessor however because empty string doesn't pass the check

if(uri) {
}

This PR explicitly checks for

if(uri!==undefined) {
}

this helps the locationType get added, and therefore not have an big app crashing error

[codecov]

Codecov Report

Merging #2444 (17a0402) into main (1946bbb) will decrease coverage by 0.05%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main    #2444      +/-   ##
==========================================
- Coverage   61.14%   61.09%   -0.06%     
==========================================
  Files         538      538              
  Lines       25079    25079              
  Branches     5857     5857              
==========================================
- Hits        15335    15322      -13     
- Misses       9426     9439      +13     
  Partials      318      318              

Impacted Files	Coverage Δ
packages/core/util/types/mst.ts	84.37% <66.66%> (ø)
...inearGenomeView/components/RefNameAutocomplete.tsx	75.26% <0.00%> (-10.76%)	⬇️
packages/core/TextSearch/TextSearchManager.ts	96.55% <0.00%> (-3.45%)	⬇️
...gins/svg/src/SvgFeatureRenderer/components/util.ts	96.07% <0.00%> (-1.97%)	⬇️
...gFeatureRenderer/components/ProcessedTranscript.js	85.13% <0.00%> (-1.36%)	⬇️
products/jbrowse-web/src/util.ts	27.27% <0.00%> (ø)

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