Revert the behavior of --diff-command wrt to non-existent files

[rgrinberg]

Do I understand correctly and this now means that the documentation in doc/concepts/promotion.rst doesn’t apply any more:

• If <file1> doesn't exist, it will compare with the empty file.

(I had been using that behaviour in some places to make sure some expected output is indeed empty; that’s easy to fix but the documentation should be fixed if that changed on purpose)

Originally posted by @shym in #13696 (comment)

[shonfeder]

Discussion has proceeded in the linked PR that introduced the breaking change that we are considering a regression. The conclusion is currently that the behavior which needs to be reverted is only

that --diff-command passes /dev/null in place of empty files

@shym has confirmed that this is what they think should be reverted, and @Julow has agreed that the new behavior with regard to empty files is acceptable.

Thus, we do not need to revert the distinction we now make between empty files and missing files (as this is a useful distinction), as attempted in #13974 (which, IMO, looks like the right fix based on what was initially specified in this issue). We only need the adjustment to the behavior of the diff-command.