Give NullParameter a friendly string representation

[yamin-oanda]

Otherwise stringifying it results in random output like com.twitter.finagle.mysql.Parameter$NullParameter$@5550f534.

This doesn't work well with e.g. snapshot tests.

[CLAassistant]

All committers have signed the CLA.

[xin301x]

这是来自QQ邮箱的假期自动回复邮件。   您好，我最近正在休假中，无法亲自回复您的邮件。我将在假期结束后，尽快给您回复。