Assimp build issue with 3.3.1

Issue introduced here when using Assimp 3.3.1: 34170af1 (comment 350649)

On mac, I now can't build with the following error:

/Users/agirault/Projects/imstk/rel/External/Assimp/src/code/D3MFImporter.cpp:230:29: error: invalid operands to binary expression ('float (*)(const char *, const char *)' and 'nullptr_t')
        vertex.z = ai_strtof>(xmlReader->getAttributeValue(D3MF::XmlTag::z.c_str()), nullptr);
                   ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Bug is here: https://github.com/assimp/assimp/blob/v3.3.1/code/D3MFImporter.cpp#L230

Not sure why dashboard are fine, for any platform.

@NickMilef @sreekanth-arikatla @jbvimort I remember talking about upgrading to Assimp 4.0.1 at some point. Do you remember the reason why we didn't?