TST: check number of returned regions

added check on `n` for all tests of label()
BUG: fix number of regions in case where no regions are found

Added special-case logic for the case were no regions were found so
that 0 is returned instead of 1

Fixes issue #3082
Merge pull request #3083 from tacaswell/bug_empty_label_3082

Bug: should return 0 if no regions found