[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/tests/FillBuffer.cpp b/libs/gui/tests/FillBuffer.cpp
index ccd674f..b60995a 100644
--- a/libs/gui/tests/FillBuffer.cpp
+++ b/libs/gui/tests/FillBuffer.cpp
@@ -93,11 +93,11 @@
     android_native_buffer_t* anb;
     ASSERT_EQ(NO_ERROR, native_window_dequeue_buffer_and_wait(anw.get(),
             &anb));
-    ASSERT_TRUE(anb != NULL);
+    ASSERT_TRUE(anb != nullptr);
 
     sp<GraphicBuffer> buf(GraphicBuffer::from(anb));
 
-    uint8_t* img = NULL;
+    uint8_t* img = nullptr;
     ASSERT_EQ(NO_ERROR, buf->lock(GRALLOC_USAGE_SW_WRITE_OFTEN,
             (void**)(&img)));
     fillRGBA8Buffer(img, buf->getWidth(), buf->getHeight(), buf->getStride());