Add support for null arrays in the blittable array marshaller's pinning optimization.

[jkoritzinsky]

Example generated code:

void Foo(int[] p)
{
   ref int __byref_p = ref (p == null ? ref System.Runtime.CompilerServices.Unsafe.NullRef<int>() : ref System.Runtime.InteropServices.MemoryMarshal.GetArrayDataReference(p));
   fixed (int* __p_gen_native= &p)
   {
        Foo__PInvoke__(__p_gen_native);
   }
}

Fixes #1051

[jkotas]

ref System.Runtime.CompilerServices.Unsafe.NullRef<int>()

Cheaper way to create a null ref in this case is ref *(int*)0. It does not require inlined method call.