Empty attribute values in styles should be strings

In aapt1, if you specified the value of an attribute in a style as an
empty string (eg. <item name="my_attr"></item>), the encoded value would
be an empty string. In aapt2 currently, @null is encoded instead. This
change restores aapt1 behavior. Use @null explicitly if the desired
value is to be @null.

Bug: 133450400
Test: manual comparison of APK created by aapt1 and aapt2
<attr name="my_attr" format="string|reference" />
<style name="MyName">
    <item name="my_attr"></item>
</style>

$ aapt p -M AndroidManifest.xml -F out1.apk -S res -f

$ aapt2 compile --dir res-o compiled.flata
$ aapt2 link --manifest AndroidManifest.xml -o out2.apk compiled.flata
$ aapt2 dump out2.apk

Change-Id: I8aa0ba30379dac0b1229da525abbc5482f40114b