Don't throw a NPE in SharedLog#e

Some DhcpServer error code paths could cause a NPE, when the logError
callback was called without an exception. Allowing SharedLog#e to be
called with a null Throwable is less error-prone.

Bug: b/109584964
Test: atest FrameworksNetTests
Change-Id: Idbcdd330a9d1951b27aaf525aaf12e52e102872c
2 files changed