t_quat::set: return a zero rotation instead of a zero quaternion

It could be argued that the default constructor for quaternion should
return a zero rotation i.e. {0, 0, 0, 1}, but I decided not to go
there.

include/vsg/maths/quat.h

@@ -98,7 +98,7 @@ namespace vsg
             if (len < epsilon)
             {
                 // ~zero length axis, so reset rotation to zero.
-                *this = {};
+                set(0, 0, 0, 1);
                 return;
             }