[adb] Modernize codebase by replacing NULL with nullptr

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

Test: m
Bug: 68236239
Change-Id: Ia8c4deacafed2f3b7dbc3d4c3c77c6c632e3de81
diff --git a/adb/client/main.cpp b/adb/client/main.cpp
index 44ed3a2..de6c723 100644
--- a/adb/client/main.cpp
+++ b/adb/client/main.cpp
@@ -89,10 +89,10 @@
     // unbuffer stdout and stderr just like if we were run at the console.
     // This also keeps stderr unbuffered when it is redirected to adb.log.
     if (is_daemon) {
-        if (setvbuf(stdout, NULL, _IONBF, 0) == -1) {
+        if (setvbuf(stdout, nullptr, _IONBF, 0) == -1) {
             fatal("cannot make stdout unbuffered: %s", strerror(errno));
         }
-        if (setvbuf(stderr, NULL, _IONBF, 0) == -1) {
+        if (setvbuf(stderr, nullptr, _IONBF, 0) == -1) {
             fatal("cannot make stderr unbuffered: %s", strerror(errno));
         }
     }