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

nixos/modules/installer/tools/nixos-option.sh

@@ -314,13 +314,13 @@ else
   # echo 1>&2 "Warning: This value is not an option."
 
   result=$(evalCfg "")
-  if names=$(attrNames "$result" 2> /dev/null); then
+  if [ ! -z "$result" ]; then
+    names=$(attrNames "$result" 2> /dev/null)
     echo 1>&2 "This attribute set contains:"
     escapeQuotes () { eval echo "$1"; }
     nixMap escapeQuotes "$names"
   else
-    echo 1>&2 "An error occurred while looking for attribute names."
-    echo $result
+    echo 1>&2 "An error occurred while looking for attribute names. Are you sure that \`$option' exists?"
   fi
 fi