Fix db_get_zoom() to support zooming on empty tables

[moodymudskipper]

closes #626

This is a small fix for a very interesting error!
In fact the error we see in linked issue is not one, it's just a message, and it's sent by RStudio, not base R.

Here's a minimal reproduction of the issue :

length.foo = function(x) stop("error!")
registerS3method("length", "foo", length.foo)
foo <- 1
attributes(foo) <- list(class = "foo")
#> Error in length.foo(obj) : error!
foo
#> [1] 1
#> attr(,"class")
#> [1] "foo"

As we see, the fooobject was properly created and modified, but the fact that the length method fails makes RStudio display the problematic message.

The guilty function is .rs.describeObject, it seems to be called after an object is created or modified in the console, and it calls length().

In our case we have length() --> length.zoomed_dm() --> tbl_zoomed() --> dm_get_zoom(), and dm_get_zoom(our_zoomed_dm) fails through abort_not_pulling_multiple_zoomed()

Our code didn't call length() on our object, so the bug is not our fault but
Rstudio's, however this showed us that our length method has a problem since
a zoomed dm containing an empty table is legal but its length isn't.

The idiom lengths(zoom) != 0 was used to filter the zoomed rows, likely expecting zero length elements to be NULL, but length(tibble()) has length zero too. We use !map_lgl(zoom, is.null) instead and the issue disappears.

I opened a RStudio issue too : rstudio/rstudio#9887

[krlmlr]

Good catch!

[krlmlr]

Thanks!