[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/GLConsumer.cpp b/libs/gui/GLConsumer.cpp
index 6237aa0..5532db4 100644
--- a/libs/gui/GLConsumer.cpp
+++ b/libs/gui/GLConsumer.cpp
@@ -405,7 +405,7 @@
     }
 
     GLC_LOGV("updateAndRelease: (slot=%d buf=%p) -> (slot=%d buf=%p)",
-            mCurrentTexture, mCurrentTextureImage != NULL ?
+            mCurrentTexture, mCurrentTextureImage != nullptr ?
                     mCurrentTextureImage->graphicBufferHandle() : nullptr,
             slot, mSlots[slot].mGraphicBuffer->handle);