Workaround null & mixed being ReservedWords

[fbourigault]

Symfony 6 added return types to most of it's signatures.

This is reported as ReservedWords by Psalm.

See vimeo/psalm#7026 and doctrine/dbal#5053 for more context.

[codecov-commenter]

Codecov Report

Merging #1817 (ef3a63c) into master (5930bad) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1817   +/-   ##
=========================================
  Coverage     87.83%   87.83%           
  Complexity     7836     7836           
=========================================
  Files           281      281           
  Lines         22748    22748           
=========================================
  Hits          19981    19981           
  Misses         2767     2767           

Flag	Coverage Δ
5-max	87.83% <ø> (ø)
7.4	87.83% <ø> (ø)
agnostic	68.54% <ø> (ø)
mysql	70.53% <ø> (ø)
pgsql	70.42% <ø> (ø)
sqlite	68.46% <ø> (ø)

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