Fix NPE in NetworkMetrics

addTcpStatsResult() in NetworkMetrics was not checking for the nullness
of the pendingSummary object and trying to add stats regardless.

This patch fixes addTcpStatsResult() so that it populates pendingSummary
if necessary, similarly to addConnectResult() and addDnsResult()

Bug: 73825228
Test: runtest frameworks-net
Change-Id: I616a4be0e6e5920bd2f5c58292fea769bd516df7