[libs/ui] Modernize codebase by replacing NULL with nullptr

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

Test: m
Bug: 68236239
Change-Id: I240d39ef4328943447149700b3ece371681078d1
diff --git a/libs/ui/Fence.cpp b/libs/ui/Fence.cpp
index ed7ccb0b..4ce891e 100644
--- a/libs/ui/Fence.cpp
+++ b/libs/ui/Fence.cpp
@@ -110,7 +110,7 @@
     }
 
     struct sync_file_info* finfo = sync_file_info(mFenceFd);
-    if (finfo == NULL) {
+    if (finfo == nullptr) {
         ALOGE("sync_file_info returned NULL for fd %d", mFenceFd.get());
         return SIGNAL_TIME_INVALID;
     }