AAPT2: Encode empty attribute string values

Due to another bug, empty strings in XML files
were encoded as NULLs. This was only needed for
encoding missing namespace URIs. Attribute
values should remain empty strings.

Bug:29939875
Bug:29462255
Change-Id: I3897661d85865c88bb2b7cf1495da16c30f7272e
3 files changed