[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/SurfaceTextureFBO_test.cpp b/libs/gui/tests/SurfaceTextureFBO_test.cpp
index 0134273..f34561f 100644
--- a/libs/gui/tests/SurfaceTextureFBO_test.cpp
+++ b/libs/gui/tests/SurfaceTextureFBO_test.cpp
@@ -39,12 +39,12 @@
     android_native_buffer_t* anb;
     ASSERT_EQ(NO_ERROR, native_window_dequeue_buffer_and_wait(mANW.get(),
             &anb));
-    ASSERT_TRUE(anb != NULL);
+    ASSERT_TRUE(anb != nullptr);
 
     sp<GraphicBuffer> buf(GraphicBuffer::from(anb));
 
     // Fill the buffer with green
-    uint8_t* img = NULL;
+    uint8_t* img = nullptr;
     buf->lock(GRALLOC_USAGE_SW_WRITE_OFTEN, (void**)(&img));
     fillRGBA8BufferSolid(img, texWidth, texHeight, buf->getStride(), 0, 255,
             0, 255);
@@ -63,7 +63,7 @@
 
         ASSERT_EQ(NO_ERROR, native_window_dequeue_buffer_and_wait(mANW.get(),
                 &anb));
-        ASSERT_TRUE(anb != NULL);
+        ASSERT_TRUE(anb != nullptr);
 
         buf = GraphicBuffer::from(anb);