Fix runtime issues with pair_exp6_rx_kokkos

   // Default constructor -- nullify everything.

   PairExp6ParamDataTypeKokkos<DeviceType>(void)

      : n(0), epsilon1(NULL), alpha1(NULL), rm1(NULL), mixWtSite1(NULL),

              epsilon2(NULL), alpha2(NULL), rm2(NULL), mixWtSite2(NULL),

              epsilonOld1(NULL), alphaOld1(NULL), rmOld1(NULL), mixWtSite1old(NULL),

              epsilonOld2(NULL), alphaOld2(NULL), rmOld2(NULL), mixWtSite2old(NULL)

      : n(0)

   {}

};

   // Default constructor -- nullify everything.

   PairExp6ParamDataTypeKokkosVect<DeviceType>(void)

      : epsilon(NULL), rm3(NULL), alpha(NULL), xMolei(NULL), epsilon_old(NULL), rm3_old(NULL),

        alpha_old(NULL), xMolei_old(NULL), fractionOFA(NULL), fraction1(NULL),

        fraction2(NULL), nMoleculesOFA(NULL), nMolecules1(NULL), nMolecules2(NULL),

        nTotal(NULL), fractionOFAold(NULL), fractionOld1(NULL), fractionOld2(NULL),

        nMoleculesOFAold(NULL), nMoleculesOld1(NULL), nMoleculesOld2(NULL),

        nTotalold(NULL)

   {}

};