fix: handle Null Dereference in RomanToInteger

[vil02]

Continuation of #5456.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.21%. Comparing base (b849cbb) to head (06b9388).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5461      +/-   ##
============================================
+ Coverage     52.17%   52.21%   +0.03%     
- Complexity     3245     3246       +1     
============================================
  Files           524      524              
  Lines         15158    15156       -2     
  Branches       2881     2881              
============================================
+ Hits           7909     7913       +4     
+ Misses         6924     6919       -5     
+ Partials        325      324       -1     

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[siriak]

Looks good, thanks!