...while capturing its standard output.
As `$()` is an invalid construct, execute nodes are not supposed to
capture the output of no command being run; but it is possible to create
empty commands such as CastToCommand(Redirection(...)) or similar.
Make this a hard error instead of an unescapable select().
This was noticed in #10432, which should now error out like so:
```
Error: Cannot capture standard output when no command is being executed
0| $(<$file)
~~~~~^^^^^^^^^
1|
```
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