lib: Fix emptyCaption property of Listing
lib: Fix emptyCaption property of Listing

Stop requiring the `emptyCaption` property. In cases like the hwinfo PCI
page, the table isn't rendered at all if there is nothing to show, so
requiring the specification of a dummy value is just busywork.

Also correct the comments, as the default really isn't "No entries". We
also don't want it to be, as that would introduce a translatable string
in vain.

Also fix the type to `proptype.node`, as e. g. the `SETroubleshootPage`
component passes a JSX object instead of a plain string.

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