base

Fix unintentional NPE in SpannableStringBuilder.

Previously, when getSpans() was called with a null parameter, an
unintentional NullPointerException would be thrown.

Now, we explicitly return an empty array of Objects.

Bug: 19805521
Change-Id: I52f3ae08c2bcf62f6a4d0c01f0fe08ad1134abb8