[libsysutils] Modernize codebase by replacing NULL with nullptr

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

Test: m
Bug: 68236239
Change-Id: I9e95daa0750f2622d1c4788941fdfae23ee61801
diff --git a/libsysutils/src/SocketClient.cpp b/libsysutils/src/SocketClient.cpp
index 971f908..0625db7 100644
--- a/libsysutils/src/SocketClient.cpp
+++ b/libsysutils/src/SocketClient.cpp
@@ -42,8 +42,8 @@
     mSocket = socket;
     mSocketOwned = owned;
     mUseCmdNum = useCmdNum;
-    pthread_mutex_init(&mWriteMutex, NULL);
-    pthread_mutex_init(&mRefCountMutex, NULL);
+    pthread_mutex_init(&mWriteMutex, nullptr);
+    pthread_mutex_init(&mRefCountMutex, nullptr);
     mPid = -1;
     mUid = -1;
     mGid = -1;
@@ -135,9 +135,9 @@
     const char *end = arg + len;
     char *oldresult;
 
-    if(result == NULL) {
+    if(result == nullptr) {
         SLOGW("malloc error (%s)", strerror(errno));
-        return NULL;
+        return nullptr;
     }
 
     *(current++) = '"';