[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/SurfaceTextureGLToGL.h b/libs/gui/tests/SurfaceTextureGLToGL.h
index 5d43a48..3a87c12 100644
--- a/libs/gui/tests/SurfaceTextureGLToGL.h
+++ b/libs/gui/tests/SurfaceTextureGLToGL.h
@@ -38,7 +38,7 @@
 
     void SetUpWindowAndContext() {
         mProducerEglSurface = eglCreateWindowSurface(mEglDisplay, mGlConfig,
-                mANW.get(), NULL);
+                mANW.get(), nullptr);
         ASSERT_EQ(EGL_SUCCESS, eglGetError());
         ASSERT_NE(EGL_NO_SURFACE, mProducerEglSurface);