Do not send 0 byte string content.

Trying to send even 0 bytes to closed socket leads to
broken pipe error. Sometimes property service is just
quick enough and closes the socket between send(valuelen)
and send(value) in the case where valuelen is 0.

Bug: http://b/34670529
Test: adb reboot 20 times and make sure phone service did not fail
Test: run bionic-unit-tests --gtest_filter=prop*
Change-Id: I96f90ca6fe1790614e7efd3015bffed1ef1e9040