(#138) TimeTrackEntry: description should not be optional

[llorllale]

(REF) TimeTrackEntry.description: now returns String instead of
          Optional<String>
(REF) EntrySpec: no longer assuming that user provided 'null' value for
          description
(REF) refactor: XmlTimeTrackEntry, XmlTimeTrackEntryTest

closes #138

[codecov]

Codecov Report

Merging #139 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #139   +/-   ##
=======================================
  Coverage   90.74%   90.74%           
=======================================
  Files          47       47           
  Lines         681      681           
  Branches       32       32           
=======================================
  Hits          618      618           
  Misses         55       55           
  Partials        8        8

Impacted Files	Coverage Δ
.../org/llorllale/youtrack/api/IssueTimeTracking.java	91.83% <100%> (ø)	⬆️
.../org/llorllale/youtrack/api/XmlTimeTrackEntry.java	92.3% <100%> (ø)	⬆️

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