Fix broken filtering through discriminator

filter.go

@@ -3,13 +3,13 @@ package mongo
 import "log"
 
 type Query struct {
-	Filter map[string]any
+	Filter map[string]any `json:"filter"`
 }
 
 type Filter struct {
-	Name string
-	Op string
-	Value string
+	Name string `json:"name"`
+	Op string `json:"op"`
+	Value string `json:"value"`
 }
 
 func makeFilter(name string, value any) *Filter {
@@ -64,7 +64,7 @@ func Mongofy(q *Query) map[string]any {
 	}
 
 	if len(conditions) == 0 {
-		return nil
+		return map[string]any{}
 	}
 
 	if logic == "or" {