docs(operators): add documentation for defaultIfEmpty

[hardikpthv]

Closes #73

[codecov-io]

Codecov Report

Merging #330 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #330   +/-   ##
=======================================
  Coverage   90.78%   90.78%           
=======================================
  Files         116      116           
  Lines         445      445           
  Branches        9        9           
=======================================
  Hits          404      404           
  Misses         40       40           
  Partials        1        1

Impacted Files	Coverage Δ
src/operator-docs/conditional/defaultIfEmpty.ts	100% <ø> (ø)	:arrow_up:

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