make keyword nullable in InboundSMS.php

[SecondeJK]

Another small PR to make keyword nullable when hydrating an InboundSMS webhook.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[codecov-commenter]

Codecov Report

Merging #440 (c92b7a1) into main (fc1e3a9) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main     #440   +/-   ##
=========================================
  Coverage     78.87%   78.87%           
- Complexity     2323     2324    +1     
=========================================
  Files           218      218           
  Lines          6252     6253    +1     
=========================================
+ Hits           4931     4932    +1     
  Misses         1321     1321           

Files	Coverage Δ
src/SMS/Webhook/InboundSMS.php	96.72% <100.00%> (+0.05%)	:arrow_up:

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