Fix bug in outer_indexer where the special case of an empty right array resulted in bogus return data.
Fixes bug described in issue #10618.

tests pls
 pls add a release note in whatsnew/0.17.0

any tests at a higher level that hit this code?
 As far as I can tell this boundary case is already tested at the higher level (https://github.com/pydata/pandas/blob/master/pandas/tests/test_index.py#L919). Those tests never hit the low level cython code though, since these boundary cases are preempted in the high-level code (e.g., https://github.com/pydata/pandas/blob/master/pandas/core/index.py#L1419).
 once you fix the testing function, pls squash, and ping.
 Done.
 thanks you sir!
