Add tests for empty RelationTable

As the title says, the MR adds a test for empty RelationTable.

Closes #528 (closed)

cc: @amathad, @sponce, @graven

Phys/FunctorCore/include/Functors/Functional.h

@@ -221,6 +221,8 @@ namespace Functors::Functional {
             using std::next;
             using std::make_pair;
 
+            if ( begin( range ) == end( range ) ) throw std::runtime_error( "Attempt to iterate over an empty range" );
+
             auto min_entry = std::accumulate(
                 next( begin( range ) ), end( range ),
                 make_pair( *begin( range ), functor( mask_arg, mask, *begin( range ), input... ) ),
@@ -264,6 +266,8 @@ namespace Functors::Functional {
             using std::next;
             using std::make_pair;
 
+            if ( begin( range ) == end( range ) ) throw std::runtime_error( "Attempt to iterate over an empty range" );
+
             auto max_entry = std::accumulate(
                 next( begin( range ) ), end( range ),
                 make_pair( *begin( range ), functor( mask_arg, mask, *begin( range ), input... ) ),