[tables] Allow slots for empty result sets

[tmorehouse]

added two slots:

• empty to be shown if no items (items.length === 0)
• emptyFiltered to be shown if a filtered result (_items.length === 0) has no records

and one prop
showEmpty to enable showing the empty slots

One method to address issue #385

There might be better ways to do this though. comments/changes welcome.

[bastones]

It is not very clear in the documentation that the showEmpty prop MUST be set on the b-table component in order for the empty and emptyFiltered slots to work. Is this the case?

I'll be happy to update the documentation to make this clearer for others, but just want to double-check this is the intended behaviour.

[emanuelmutschlechner]

I think documentation is fine. show-empty is listed in the "Component Reference" of <b-table> and its usage is shown in the "Complete Example".

[bastones]

Is it too much hard work to add a few words to the row in the table under the "Slots" heading to make developers aware the empty and emptyfiltered slots won't work if the show-empty prop is not set?

I will be happy to do this myself and submit a pull request, but I stand by what I said - it is not clear and good documentation should be clear about technical behaviour. It is only after using the component and wasting time tracking down why the slot wasn't working that I now know why this happens: it's because I can see there is a default message that gets displayed which the empty-text prop or empty can replace, but again only if the show-empty prop is set.

[arippberger]

+1 for @bastones comment on adding info to the documentation. It doesn't make it clear show-empty needs to be set to true.