fix: make translation module type check correctly

[kristw]

🐛 Bug Fix

• type of singleton can be undefined before the first configure() or getInstance() call.

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[codecov]

Codecov Report

Merging #140 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #140   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          77     77           
  Lines         988    988           
  Branches      238    238           
=====================================
  Hits          988    988

Impacted Files	Coverage Δ
...superset-ui-translation/src/TranslatorSingleton.ts	100% <100%> (ø)	⬆️

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