Fix VLE.set_PH when (vle) chemicals exist

[yalinli2]

@yoelcortes
Previously, when NoEquilibrium was raised for no chemicals/vle chemicals, self._N (would be 0 for these cases) was not set, so set_PH does resolve to the correct fix below:

        if self._N == 0:
            thermal_condition.T = self.mixture.xsolve_T_at_HP(
                self._phase_data, H, thermal_condition.T, P
            )
            return

Note: the following block does set thermal_condition and thermal_condition.P, but not thermal_condition.T

        elif P_spec:
            if V_spec:
                try:
                    self.set_PV(P, V, gas_conversion, liquid_conversion, wt=wt)
                except NoEquilibrium:
                    thermal_condition = self._thermal_condition
                    thermal_condition.P = P
            elif H_spec:
                try:
                    self.set_PH(P, H, gas_conversion, liquid_conversion)
                except NoEquilibrium:
                    thermal_condition = self._thermal_condition
                    thermal_condition.P = P

Similar issues might exist for setup_PS as well, thanks.

[yoelcortes]

Thanks for submitting this issue, @yalinli2. I took care of it in 10afe9f. I removed instances of raising NoEquilibrium error within setup so that set_XY methods handle the special case of no equilibrium chemicals. Thanks for all the helpful details too.