Add a soft return type to AbstractDocument::jsonSerialize

[mpdude]

This helps to avoid the following deprecation message being generated by Symfony's DebugClassLoader:

Method "JsonSerializable::jsonSerialize()" might add "mixed" as a native return type declaration in the future. Do the same in implementation "Solarium\Core\Query\AbstractDocument" now to avoid errors or add an explicit @return annotation to suppress this message.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.19%. Comparing base (80e85f5) to head (5d513a4).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1171      +/-   ##
==========================================
+ Coverage   98.16%   98.19%   +0.02%     
==========================================
  Files         404      404              
  Lines       10632    10633       +1     
==========================================
+ Hits        10437    10441       +4     
+ Misses        195      192       -3     

Flag	Coverage Δ
unittests	98.19% <ø> (+0.02%)	⬆️

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