Replace NULL macros with nullptr literals.

Change-Id: I918c40879aa547438f77e7d1a95fa2aa33bec398
diff --git a/cpu_ref/rsCpuScript.h b/cpu_ref/rsCpuScript.h
index c5fc183..78111ea 100644
--- a/cpu_ref/rsCpuScript.h
+++ b/cpu_ref/rsCpuScript.h
@@ -59,7 +59,7 @@
 
     bool init(char const *resName, char const *cacheDir,
               uint8_t const *bitcode, size_t bitcodeSize, uint32_t flags,
-              char const *bccPluginName = NULL);
+              char const *bccPluginName = nullptr);
     virtual void populateScript(Script *);
 
     virtual void invokeFunction(uint32_t slot, const void *params, size_t paramLength);