[ML] boost::function cannot be initialised with nullptr (#47)

This fixes a compilation failure introduced backporting the 0 -> nullptr changes.
The 7.0 branch uses std::function which has a conversion from nullptr boost::function does not

Author: davidkyle

include/maths/CPeriodicityHypothesisTests.h

@@ -257,7 +257,7 @@ class MATHS_EXPORT CPeriodicityHypothesisTests {
         };
 
     public:
-        explicit CNestedHypotheses(TTestFunc test = nullptr);
+        explicit CNestedHypotheses(TTestFunc test = TTestFunc());
 
         //! Set the null hypothesis.
         CBuilder null(TTestFunc test);