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/PppController.cpp b/server/PppController.cpp
index 19b50cb..80a36e9 100644
--- a/server/PppController.cpp
+++ b/server/PppController.cpp
@@ -93,7 +93,7 @@
         // TODO: Deal with pppd bailing out after 99999 seconds of being started
         // but not getting a connection
         if (execl("/system/bin/pppd", "/system/bin/pppd", "-detach", dev, "115200",
-                  lr, "ms-dns", d1, "ms-dns", d2, "lcp-max-configure", "99999", (char *) NULL)) {
+                  lr, "ms-dns", d1, "ms-dns", d2, "lcp-max-configure", "99999", (char *) nullptr)) {
             ALOGE("execl failed (%s)", strerror(errno));
         }
         free(lr);
@@ -116,7 +116,7 @@
 
     ALOGD("Stopping PPPD services on port %s", tty);
     kill(mPid, SIGTERM);
-    waitpid(mPid, NULL, 0);
+    waitpid(mPid, nullptr, 0);
     mPid = 0;
     ALOGD("PPPD services on port %s stopped", tty);
     return 0;