Fix NPE when parsing invalid timestamp variable inputs

[igdianov]

by raising CoersingParseValueException with detailed message for troubleshooting

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.85%. Comparing base (14c234e) to head (c7d9ae6).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #518      +/-   ##
============================================
+ Coverage     77.82%   77.85%   +0.03%     
  Complexity     1303     1303              
============================================
  Files            79       79              
  Lines          5768     5767       -1     
  Branches        761      761              
============================================
+ Hits           4489     4490       +1     
+ Misses          940      939       -1     
+ Partials        339      338       -1     

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