[logd] Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: I15ccb9cfc9967dae4320d9690f5097bc2f7d5bfe
diff --git a/logd/FlushCommand.cpp b/logd/FlushCommand.cpp
index 70ecbe0..658e079 100644
--- a/logd/FlushCommand.cpp
+++ b/logd/FlushCommand.cpp
@@ -36,7 +36,7 @@
 // reference counts are used to ensure that individual
 // LogTimeEntry lifetime is managed when not protected.
 void FlushCommand::runSocketCommand(SocketClient* client) {
-    LogTimeEntry* entry = NULL;
+    LogTimeEntry* entry = nullptr;
     LastLogTimes& times = mReader.logbuf().mTimes;
 
     LogTimeEntry::wrlock();