By MOT, the bond orders of both are 2.5. Now acc to MOT only, the compound having more electrons in antibonding orbitals will form a weaker bond and a longer bond. If their bond orders are equal. ${\mathrm{N}}^{2-}$ has $5{\mathrm{e}}^{-}$ in antibonding while ${\mathrm{N}}^{2+}$ has 4. So, ${\mathrm{N}}^{2+}$ will make a stronger bond and will have a shorter bond length.