jmespath exclude_empty fix

[DylanWelzel]

A KeyError was being raised in the transformation logic whenever a list item did not contain the target_field. In particular, this occurred when making a query to mychem.info with jmespath_exclude_empty=true, causing the transformation step to attempt dictionary access on a missing key.

curl --location --globoff 'https://mychem.info/v1/query?size=1000&fields=drugcentral.bioactivity%2Cdrugcentral.xrefs.umlscui%2Cdrugcentral.synonyms&jmespath_exclude_empty=true&always_list=drugcentral.bioactivity&jmespath=drugcentral.bioactivity%7C[%3F!action_type%20%20%26%26%20length(uniprot[%3Funiprot_id%3D%3D%27P29274%27])%20%3E%20%600%60]' \
--header 'Content-Type: application/json' \
--data '{
    "q": ["P29274"],
    "scopes": "drugcentral.bioactivity.uniprot.uniprot_id"
}'

This query would raise a KeyError: 'bioactivity' if any list item did not include "bioactivity" as a key.

See #368