Modernize codebase by replacing NULL with nullptr

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

Test: m
Bug: 68236239
Change-Id: I226a0599db4f7c3557e55cade7869d00bd314949
diff --git a/server/NetlinkCommands.cpp b/server/NetlinkCommands.cpp
index 4e940d4..473cac0 100644
--- a/server/NetlinkCommands.cpp
+++ b/server/NetlinkCommands.cpp
@@ -185,7 +185,7 @@
             .rtm_family = static_cast<uint8_t>(family),
         };
         iovec iov[] = {
-            { NULL,  0 },
+            { nullptr,  0 },
             { &rule, sizeof(rule) },
         };
         uint16_t flags = NETLINK_DUMP_FLAGS;