Treat "" as ObjectId value equivalent to nil (#23580)

This is to enable is-empty and non-empty tests on :type/MongoBSONID columns.

modules/drivers/mongo/src/metabase/driver/mongo/query_processor.clj

@@ -245,9 +245,10 @@
   [[_ value {base-type :base_type}]]
   (if (and (isa? base-type :type/MongoBSONID)
            (some? value))
-    ;; Passing a nil to the ObjectId constructor throws an exception
+    ;; Passing nil or "" to the ObjectId constructor throws an exception
     ;; "invalid hexadecimal representation of an ObjectId: []" so, just treat it as nil
-    (ObjectId. (str value))
+    (when (not= value "")
+      (ObjectId. (str value)))
     value))
 
 (defn- $date-from-string [s]

modules/drivers/mongo/test/metabase/driver/mongo_test.clj

@@ -286,7 +286,20 @@
             (rows (mt/dataset with-bson-ids
                     (mt/run-mbql-query birds
                       {:filter [:is-null $bird_id]}))))
-         "handle null ObjectId queries properly (#11134)"))))
+         "handle null ObjectId queries properly (#11134)")
+
+     (is (= [[3 "Unlucky Raven" nil]]
+            (rows (mt/dataset with-bson-ids
+                    (mt/run-mbql-query birds
+                      {:filter [:is-empty $bird_id]}))))
+         "treat null ObjectId as empty (#15801)")
+
+     (is (= [[1 "Rasta Toucan" (ObjectId. "012345678901234567890123")]
+             [2 "Lucky Pigeon" (ObjectId. "abcdefabcdefabcdefabcdef")]]
+            (rows (mt/dataset with-bson-ids
+                    (mt/run-mbql-query birds
+                      {:filter [:not-empty $bird_id]}))))
+         "treat non-null ObjectId as not-empty (#15801)"))))
 
 (deftest bson-fn-call-forms-test
   (mt/test-driver :mongo