Add BigNumber::of(Nullable)Untrusted()

[BenMorel]

Fixes #115.

[codecov]

Codecov Report

❌ Patch coverage is 97.43590% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.01%. Comparing base (a89bc96) to head (f8c477b).

Files with missing lines	Patch %	Lines
src/BigNumber.php	96.87%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #116      +/-   ##
============================================
- Coverage     99.05%   99.01%   -0.04%     
- Complexity      779      797      +18     
============================================
  Files            21       21              
  Lines          1901     1938      +37     
============================================
+ Hits           1883     1919      +36     
- Misses           18       19       +1     

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