Improve handling NullpointerException by APathQuery

[EBrader]

Improves user feedback corresponding to #741

There were 2 places that could by relative 'simple' paths result in NullpointerException.
This PR makes the Throwable more clear to the user.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.23%. Comparing base (45861a6) to head (6160595).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #789      +/-   ##
============================================
+ Coverage     72.21%   72.23%   +0.01%     
- Complexity     7211     7214       +3     
============================================
  Files           680      680              
  Lines         23387    23393       +6     
  Branches       3789     3791       +2     
============================================
+ Hits          16890    16898       +8     
+ Misses         4756     4755       -1     
+ Partials       1741     1740       -1     

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