feat(coarse_reverse): ignore empty abbr properties from PIP service

[missinglink]

Following on from #1003 ...

In 1003 I added code to try/catch the document construction code (to avoid the service crashing), thanks to @gitsno I was able to see that the true culprits are responses like this:

"id":85632733,
"name":"Somaliland",
"abbr":""

This PR ignores empty abbr attributes (ie: abbr='') and so will not cause a fatal error now unless the name itself is invalid.

As a result, the API will no longer return an empty resultset for cases where the PIP service returns results with an empty abbr property.

closes #1009

[trescube]

good catch!