Add handling for empty Jsonb to fix #1649 (#1650)

dialects/postgres/postgres.go

@@ -63,6 +63,9 @@ type Jsonb struct {
 
 // Value get value of Jsonb
 func (j Jsonb) Value() (driver.Value, error) {
+	if len(j.RawMessage) == 0 {
+		return nil, nil
+	}
 	return j.MarshalJSON()
 }