Replace use of nullptr in headers with NULL

include/assimp/anim.h

@@ -377,9 +377,9 @@ struct aiAnimation {
     : mDuration(-1.)
     , mTicksPerSecond(0.)
     , mNumChannels(0)
-    , mChannels(nullptr)
+    , mChannels(NULL)
     , mNumMeshChannels(0)
-    , mMeshChannels(nullptr) {
+    , mMeshChannels(NULL) {
         // empty
     }
 

include/assimp/metadata.h

@@ -187,7 +187,7 @@ struct aiMetadata {
     static inline
     aiMetadata *Alloc( unsigned int numProperties ) {
         if ( 0 == numProperties ) {
-            return nullptr;
+            return NULL;
         }
 
         aiMetadata *data = new aiMetadata;