fix #22354 - aa[key] ~= null discarded

[rainers]

don't implicitly cast rhs to value-type for op=

[dlang-bot]

Thanks for your pull request, @rainers!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + dmd#22361"

[thewilsonator]

cc @dkorpel please make sure this gets in the release

[dkorpel]

2.112.0 has already been released (it's just waiting for the website to deploy, which has been stuck since October). This will be in 2.112.1