Enable RedundantNullCheck by default

[wilkinsona]

With thanks to @nosan for the pointer to the check, we should explore enabling Error Prone's RedundantNullCheck by default. Depending on how disruptive that is, we may want to provide a configuration option to easily turn it off on a per-project basis. This would ease gradual adoption.

[wilkinsona]

Unfortunately, the automated check doesn't work well for a library where consumers may not have null checking in place. We got lots of false positives for redundant checks. Correcting those would hurt usability as we'd end up with more NPEs, perhaps some distance from where the null was actually passed in.