[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/GLTest.h b/libs/gui/tests/GLTest.h
index f0d27a8..f290b3c 100644
--- a/libs/gui/tests/GLTest.h
+++ b/libs/gui/tests/GLTest.h
@@ -39,7 +39,7 @@
             mEglDisplay(EGL_NO_DISPLAY),
             mEglSurface(EGL_NO_SURFACE),
             mEglContext(EGL_NO_CONTEXT),
-            mGlConfig(NULL) {
+            mGlConfig(nullptr) {
     }
 
     virtual void SetUp();