Fixed NumericNode behavior with empty/nil text values

When a numeric node finds an empty value at its configured path, it should not throw if a default value had been provided.
this will be XML::MappingError rather than RuntimeError if you raise NoAttrValueSet as described above. If we want to check for the exception message too, I believe the correct method would be assert_raise_with_message. (but the other tests don't check exception messages right now -- should we start doing it now?)