specifying a `[[resource.config]]` `requires` as a string doesn't raise an error

[grondo]

I had inadvertently created a config with:

[[resource.config]]
hosts = "pi[0-2]"
cores = "0-3"
properties = "cm4"

This is an error since properties is not an array, but the value is ignored and no error is returned on flux config [re]load.