Missing: add empty vector overload

testing with non-existing shared field in the variant_reader_test. Not
ideal, but there is really no real case where we want to test an empty
vector return value. This is more of a thing for the posteriority I
would say.

fixes #106 
DESK -> DESC. Also, could we add missing() tests for the other *_shared_field() functions (just query non-existent fields)