Uniformly handle `null` arguments in java.util.Formatter.
Uniformly handle `null` arguments in java.util.Formatter.

Now that error conditions are uniformly handled for all conversions
in `j.u.Formatter`, we can also uniformly handle `null` arguments.

The only exception is for the `b`/`B` conversions, which have a
dedicated code path.

scala-js/scala-js,scala-js/scala-js,sjrd/scala-js,gzm0/scala-js,scala-js/scala-js,sjrd/scala-js,gzm0/scala-js,sjrd/scala-js