Modernize codebase by replacing NULL with nullptr

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

Test: m
Bug: 68236239
Change-Id: I8cb04f9f99be029d7fef13c4c7c77aea515f3d61
diff --git a/libhidlcache/MemoryDealer.cpp b/libhidlcache/MemoryDealer.cpp
index e9196a1..dea7bc7 100644
--- a/libhidlcache/MemoryDealer.cpp
+++ b/libhidlcache/MemoryDealer.cpp
@@ -240,7 +240,7 @@
             return freed;
         }
     }
-    return 0;
+    return nullptr;
 }
 
 void SimpleBestFitAllocator::dump(const char* tag) const {