fix: handle Null Dereference in NthUglyNumber

[vil02]

Continuation of #5456.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.30%. Comparing base (1460eb7) to head (22cd14b).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5469   +/-   ##
=========================================
  Coverage     52.30%   52.30%           
  Complexity     3256     3256           
=========================================
  Files           525      525           
  Lines         15183    15184    +1     
  Branches       2887     2887           
=========================================
+ Hits           7941     7942    +1     
  Misses         6919     6919           
  Partials        323      323           

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[BamaCharanChhandogi]

Looks good, thanks.