[opengl] Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: Ia2f91707b277954d5105f6fdb489caad9ef8848c
diff --git a/opengl/libs/EGL/egl_display.h b/opengl/libs/EGL/egl_display.h
index 79a9f08..f764028 100644
--- a/opengl/libs/EGL/egl_display.h
+++ b/opengl/libs/EGL/egl_display.h
@@ -160,7 +160,7 @@
     const egl_display_t* get() const { return mDpy; }
           egl_display_t* get()       { return mDpy; }
 
-    operator bool() const { return mDpy != NULL; }
+    operator bool() const { return mDpy != nullptr; }
 
 private:
     egl_display_t* mDpy;