Fix DateTimeImmutable::sub() to not return false

[AndrolGenhald]

@Ocramius The ->sub() issue is separate from this, this issue is only for ->modify(), but I do believe ->sub() was incorrect in your PR, here's an example for that special case you found: https://3v4l.org/TIWRB

It indeed gives a warning and returns a clone, so there's no false return.

Also, go get some sleep :stuck_out_tongue:

Originally posted by @AndrolGenhald in https://github.com/vimeo/psalm/issues/8454#issuecomment-1238386327

[psalm-github-bot[bot]]

Hey @AndrolGenhald, can you reproduce the issue on https://psalm.dev ?