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 
Rewriting them should not change the result -- missing(get non-existent field) should still return true. We should add the tests now before we forget, since this is the kind of thing that won't show up in coverage stats.