ExifInterface: Handle test behavior

This CL corrects the compare values of lg_g4_iso_800 test file, which
was originally default values. Also, since .xml files do not handle
empty string cases, this CL adds code to treat it equal to a null
value.

Bug: 29409825
Change-Id: I476269df494473667143ef5ebb9a46be8ffcba3e
2 files changed