Avoid "android.media.mediacodec.log-session-id" print redundancy

set "log-session-id" media metrics only if it is not empty.

Test: adb shell dumpsys media.metrics | grep codec
Change-Id: I6c2fc61c587feb5d9dbb431aaa10157cb629866c
Signed-off-by: guochuang <guochuang@xiaomi.corp-partner.google.com>