BUGFIX strtoupper null value deprecation

[muskie9]

resolves #61

[codecov]

Codecov Report

Base: 40.36% // Head: 40.11% // Decreases project coverage by -0.24% ⚠️

Coverage data is based on head (5648be7) compared to base (47e3832).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.0      #62      +/-   ##
============================================
- Coverage     40.36%   40.11%   -0.25%     
- Complexity       54       55       +1     
============================================
  Files             4        4              
  Lines           166      167       +1     
============================================
  Hits             67       67              
- Misses           99      100       +1     

Flag	Coverage Δ
unittests	40.11% <0.00%> (-0.25%)	⬇️

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Impacted Files	Coverage Δ
src/AddressDataExtension.php	57.26% <0.00%> (-0.50%)	⬇️

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