Prepare example app showcasing the package usage

[rszymanski]

Description: We want to provide use cases of the package, examples that come to mind:

1. Filtering a table that returns 0 rows
2. A plot that doesn't show any data due to the underlying data being filtered and having 0 rows
3. Nothing appearing on the screen until a user uploads a file

[dsen6644]

Would be very helpful to have these situations documented with examples. Might also valuable to show circumstances where the table has more than 0 rows but less than a specified amount of rows (i.e not enough data vs no data), or something tied to the nativevalidate or need functions.

[rszymanski]

@dsen6644 Thank your for the suggestion, we'll keep that in mind during the next batch of work on shiny.emptystate!