Add a test for empty content in BinaryContentTagger.

[wiarlawd]

Other changes:

• Use assertArrayEquals instead of assertTrue with Arrays.equals.
• Always use UTF_8 with getBytes, to avoid findBugs warnings.

Change-Id: Ib226525ebe61fae12b0885f7ab6d625a8fe574af (cherry picked from commit 089b95cc844fe674f4c41f3c28713415fa95cd03)