code: public function null_eq(string $name, $arg)
      {
          if ($arg === null) {
              $this->sql .= "$name is null";
          }
          else {
              $this->args[] = $arg;
              $argNum = count($this->args);
              $this->sql .= "$name = \$$argNum";
          }
          return $this;
      }
docs: handle null argument for correct sql
  @param string $name
  @param $arg
  @return $this