chore: use nullptr where possible in place of NULL

src/lib/base/String.cpp

@@ -109,7 +109,7 @@ std::string sprintf(const char *fmt, ...)
   char *buffer = tmp;
   int len = (int)(sizeof(tmp) / sizeof(tmp[0]));
   std::string result;
-  while (buffer != NULL) {
+  while (buffer != nullptr) {
     // try printing into the buffer
     va_list args;
     va_start(args, fmt);
@@ -131,7 +131,7 @@ std::string sprintf(const char *fmt, ...)
       if (buffer != tmp) {
         delete[] buffer;
       }
-      buffer = NULL;
+      buffer = nullptr;
     }
   }