I got the same problem ! the complier does not kown the function !
Came across this question when I also got stuck on this compiler error.
I figured out that it's because of the deployment target version. iOS 8 is delivered with Metal 1.0, iOS 9 with Metal 1.1, iOS 10 with Metal 1.2. As stated in Metal Shading Language Specification, null texture functions are supported since Metal 1.2. So if the deployment target version is iOS 9.x or older, is_null_texture is not included in building process.
I got the same error for float2x2(float, float, float, float) which is included since Metal 2.0 (iOS 11).