Fix usage of assertThat in assertReviewers

Instead of:

 assertThat(iterable.isEmpty()).isTrue()

use:

 assertThat(iterable).isEmpty()

Also add a cast to prevent deprecation warning that was introduced
in version 0.25 of the Truth library.

Change-Id: I551229051af3c87fe7031f4f0f6433ef8e673fe6
1 file changed