While I applaud perf increases, I have seen my fair share real life cases of caching systems which gave errors when NULL cache keys were being generated. The error was iirc thrown by the memcached adapter, and quite a pain to troubleshoot, as it carried no info whatsoever as to the offending bit of code.
Of course, the data sets used in dev or for ci and testing never had any data that would trigger the same problem, making it hard to spot before going live / using real data.
In the end, I think we had to resort to modifying the bit of code which made the memcache call, adding in there an is_null check and logging the whole stack trace, just to get started on troubleshooting.
If I get it correctly, It seems that this commit is bringing back that possible problem.
Could we have best of both worlds?

@gggeek I'd be fine throwing aTypeError when null is passed since that's the planned behavior for psr/cache v2 with PHP8 - aka what we'll do in Symfony 6.

Would you mind sending a PR doing so ?