bug#30938: 27.0; `dired-do-create-files' etc.: do NOT always raise error if no files

[Drew Adams]

> Skimming this bug report, it seems like the conclusion was that we
> don't want to change anything here, so I'm closing this bug report.

Very unfortunate.

For the record, and for Someone who might care to
fix this mistake in the future, here's a summary,
with text from previous messages.
___

There's no reason to assume that interactive and
non-interactive use of a given command should have
the same behavior wrt `dired-get-marked-files' arg
ERROR. Such hardcoding provides no benefit and can
get in the way.

A non-interactive use case for an arbitrary command
that calls ` dired-get-marked-files' does not
necessarily have `user-error' as the right behavior
for an empty set of marked files.

A common use case where the error can kick in is
when a FILTER passed to `dired-get-marked-files'
results in an empty file list.

While it is typically a user error if nothing is
marked and point is not on any file line, it is
not necessarily an error when neither of those is
the case - even for an interactive case.  The
individual command needs to decide whether that
is an error case.

Similarly, for a command B that _invokes_ such a
command A.  It's up to _B to decide_ whether to
raise an error.  If A systematically causes
`dired-get-marked-files' to raise an error in a
context where an error makes sense for A, that
can conflict with what makes sense for B.