Do not serialize empty text in manifest proto

When linking an APK in the proto format, the manifest is currently
serializes text nodes that only contain whitespace:

child: {
  text: "\n        "
  source: {
    line_number  : 0x0000000f
    column_number: 0x0000002f
  }
}

These whitespace bloat the proto size unnecessarily. Do not write these
text nodes for proto apks.

Bug: 118800653
Test: make aapt2_tests
Change-Id: Icfaaf88976f81450bbf51610a316b336deeae60c