liblog: test: android_logger_get_ "kernel" log buffer can be empty

(cherry picked from commit 37eaefa9)

If user build and for the "kernel" log buffer, allow an empty report.

Test: CTS: User: liblog#android_logger_get_
Test: remove ro.debuggable line from /default.prop,
      run liblog-unit-tests --gtest_filter=*.android_logger_get_
Bug: 62030309
Change-Id: I5f91af07608988f322b3c46574becc308e1d8b13