nixos-option: don't abort with shell failures if options are not existant

`nixos-option` basically handles two cases: the given option is either a
valid option defined using `mkOption` or an attribute set which contains
a set of options.

If none of the above cases is valid, `$1` is invalid. Unfortunatley the
script interpreted invalid options as an attribute set which rendered
shell failures when trying to evaluate the arguments.

First of all, `if names=$(attrNames ...)` resulted in `<PRIMOP>` as
`attrNames` simply evaluated `builtins.attrNames $result` which results
in a non-applied function with `$result` being empty. Trying to map over
this string using `nixMap` while applying `escapeQuotes` causes the bash
error as `eval echo "<PRIMOP>"` is invalid syntax.

Explicitly checking if `$result' contains a value (do we have an
attribute set?) and otherwise returning a warning and asking if $option
exists fixes the problem.

Fixes #48060