[gui] Modernize codebase by replacing NULL with nullptr

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

Test: m
Bug: 68236239
Change-Id: I7b39bde2d13690d46d96ae0e0737d19b11a1da9a
diff --git a/libs/gui/SurfaceComposerClient.cpp b/libs/gui/SurfaceComposerClient.cpp
index d2b472b..fc076d2 100644
--- a/libs/gui/SurfaceComposerClient.cpp
+++ b/libs/gui/SurfaceComposerClient.cpp
@@ -60,7 +60,7 @@
     while (getService(name, &mComposerService) != NO_ERROR) {
         usleep(250000);
     }
-    assert(mComposerService != NULL);
+    assert(mComposerService != nullptr);
 
     // Create the death listener.
     class DeathObserver : public IBinder::DeathRecipient {
@@ -83,7 +83,7 @@
     Mutex::Autolock _l(instance.mLock);
     if (instance.mComposerService == nullptr) {
         ComposerService::getInstance().connectLocked();
-        assert(instance.mComposerService != NULL);
+        assert(instance.mComposerService != nullptr);
         ALOGD("ComposerService reconnected");
     }
     return instance.mComposerService;