Avoid using @Nullable to annotate arrays

Avoid using @Nullable to annotate arrays

As described in the javadoc for org.eclipse.jgit.annotations.Nullable:

  Warning: Please do not use this annotation on arrays. Different
  annotation processors treat `@Nullable Object[]` differently: some
  treat it as an array of nullable objects, for consistency with
  versions of `Nullable` defined with `@Target TYPE_USE`, while others
  treat it as a nullable array of objects. JGit therefore avoids using
  this annotation on arrays altogether.

  See the checker-framework manual[1] for details.

  [1] http://types.cs.washington.edu/checker-framework/current/checker-framework-manual.html#faq-array-syntax-meaning

Change-Id: I14ffcf80adbb8145d797998de2f2fa6ab84c3ae3
Signed-off-by: Jonathan Nieder <jrn@google.com>