Simplify API logic with new select-non-nil-keys and select-keys-when fns

resources/migrations/000_migrations.yaml

src/metabase/api/card.clj

src/metabase/api/common.clj

src/metabase/api/dashboard.clj

src/metabase/api/field.clj

src/metabase/api/metric.clj

src/metabase/api/segment.clj

src/metabase/models/dashboard.clj

src/metabase/models/metric.clj

src/metabase/models/segment.clj

src/metabase/util.clj

test/metabase/api/card_test.clj

test/metabase/api/metric_test.clj

test/metabase/models/metric_test.clj

test/metabase/permissions_collection_test.clj

test/metabase/util_test.clj