[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.h b/libs/gui/tests/SurfaceTextureFBO.h
index 7f1ae84..70f988d 100644
--- a/libs/gui/tests/SurfaceTextureFBO.h
+++ b/libs/gui/tests/SurfaceTextureFBO.h
@@ -34,7 +34,7 @@
         glGenTextures(1, &mFboTex);
         glBindTexture(GL_TEXTURE_2D, mFboTex);
         glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, getSurfaceWidth(),
-                getSurfaceHeight(), 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
+                getSurfaceHeight(), 0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr);
         glBindTexture(GL_TEXTURE_2D, 0);
         ASSERT_EQ(GLenum(GL_NO_ERROR), glGetError());