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 
But they will still be testing an aspect of missing() that should be covered but is not reflected in coverage reports. If we don't write it now we will probably forget. Should be just one line of test per *_shared function -- you could have been done by now instead of having this debate :)