Fix sync type inference for MongoDB completely null fields

src/metabase/driver/mongo.clj

@@ -106,12 +106,12 @@
                                  (find-nested-fields field-value nested-fields)
                                  nested-fields)))))
 
-(s/defn ^:private most-common-object-type :- Class
+(s/defn ^:private ^Class most-common-object-type :- (s/maybe Class)
   "Given a sequence of tuples like [Class <number-of-occurances>] return the Class with the highest number of
   occurances. The basic idea here is to take a sample of values for a Field and then determine the most common type
   for its values, and use that as the Metabase base type. For example if we have a Field called `zip_code` and it's a
   number 90% of the time and a string the other 10%, we'll just call it a `:type/Number`."
-  [field-types :- [(s/pair Class "Class", s/Int "Int")]]
+  [field-types :- [(s/pair (s/maybe Class) "Class", s/Int "Int")]]
   (->> field-types
        (sort-by second)
        last

test/metabase/driver/mongo_test.clj

@@ -6,6 +6,7 @@
              [driver :as driver]
              [query-processor :as qp]
              [query-processor-test :refer [rows]]]
+            [metabase.driver.mongo :as mongo]
             [metabase.driver.mongo.query-processor :as mongo-qp]
             [metabase.models
              [field :refer [Field]]
@@ -237,3 +238,14 @@
                                              :collection "venues"}
                                   :type     :native
                                   :database (data/id)}))))
+
+
+;; tests for `most-common-object-type`
+(expect
+  String
+  (#'mongo/most-common-object-type [[Float 20] [Integer 10] [String 30]]))
+
+;; make sure it handles `nil` types correctly as well (#6880)
+(expect
+  nil
+  (#'mongo/most-common-object-type [[Float 20] [nil 40] [Integer 10] [String 30]]))