[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/MultiTextureConsumer_test.cpp b/libs/gui/tests/MultiTextureConsumer_test.cpp
index 3a25ac5..7d3d4aa 100644
--- a/libs/gui/tests/MultiTextureConsumer_test.cpp
+++ b/libs/gui/tests/MultiTextureConsumer_test.cpp
@@ -47,7 +47,7 @@
         GLTest::TearDown();
     }
     virtual EGLint const* getContextAttribs() {
-        return NULL;
+        return nullptr;
     }
     virtual EGLint const* getConfigAttribs() {
         static EGLint sDefaultConfigAttribs[] = {
@@ -105,7 +105,7 @@
 
     glClear(GL_COLOR_BUFFER_BIT);
     for (int i=0 ; i<8 ; i++) {
-        mSurface->lock(&buffer, NULL);
+        mSurface->lock(&buffer, nullptr);
         memset(buffer.bits, (i&7) * 0x20, buffer.stride * buffer.height * 4);
         mSurface->unlockAndPost();