Replace NULL macros with nullptr literals.

Change-Id: I918c40879aa547438f77e7d1a95fa2aa33bec398
diff --git a/cpp/Sampler.cpp b/cpp/Sampler.cpp
index 767d626..bf99125 100644
--- a/cpp/Sampler.cpp
+++ b/cpp/Sampler.cpp
@@ -56,10 +56,10 @@
 }
 
 #define CREATE_SAMPLER(N, MIN, MAG, WRAPS, WRAPT) sp<const Sampler> Sampler::N(sp<RS> rs) { \
-        if (rs->mSamplers.N == NULL) {                                  \
+        if (rs->mSamplers.N == nullptr) {                                \
             rs->mSamplers.N = (create(rs, MIN, MAG, WRAPS, WRAPT, 0.f)); \
-        }                                                               \
-        return rs->mSamplers.N;                                         \
+        }                                                                \
+        return rs->mSamplers.N;                                          \
     }
 
 CREATE_SAMPLER(CLAMP_NEAREST, RS_SAMPLER_CLAMP, RS_SAMPLER_CLAMP, RS_SAMPLER_NEAREST, RS_SAMPLER_NEAREST);