[input] Modernize codebase by replacing NULL with nullptr

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

Test: m
Bug: 68236239
Change-Id: I8882234efd2a0b3ef27472d3f5d4e9c69c6e7b37
diff --git a/libs/input/InputDevice.cpp b/libs/input/InputDevice.cpp
index 4287abe..5d27bf6 100644
--- a/libs/input/InputDevice.cpp
+++ b/libs/input/InputDevice.cpp
@@ -175,7 +175,7 @@
             return ⦥
         }
     }
-    return NULL;
+    return nullptr;
 }
 
 void InputDeviceInfo::addSource(uint32_t source) {