Avoid error when using facet.args = list(free = TRUE) without facets

[zeileis]

Fixes #553

In case of no facets, we should be able to just use the full data for xfree/yfree:

xfree = if (!is.null(facet)) split(c(x, xmin, xmax), facet)[[ii]] else c(x, xmin, xmax)
yfree = if (!is.null(facet)) split(c(y, ymin, ymax), facet)[[ii]] else c(y, ymin, ymax)

Then the following two yield equivalent output:

tinyplot(bill_len ~ bill_dep, data = penguins)
tinyplot(bill_len ~ bill_dep, data = penguins, facet.args = list(free = TRUE))