This was an issue with "--no-execute", which has no variables and
therefore no $HOME:
```fish
fish --no-execute /path/to/file
```
would say the error is in `~/path/to/file`.
Instead, since this is just for a message, we simply return the
filename without doing the replacement.
Fixes#10171
It used to error out when a command wasn't known, even when it was a
function that would only be discovered via autoloading.
Now we just accept that a command doesn't exist when no-execute is
given - we're not gonna execute it anyway.
Also, in the same breath stop counting empty commands after expansion
and empty wildcard expansions as errors - these depend on runtime
values, so we can't verify them without executing.
Fixes#977.
(note that it still executes "time", but that's another commit)
