use NaN for un-set threshold

[williamspatrick]

This commit in phosphor-dbus-interfaces defined an un-set threshold to be required to be NaN instead of 0. This allows thresholds to have zero values.

openbmc/phosphor-dbus-interfaces@2c33a89

As best I can tell, this is the code in phosphor-nvme that determines the threshold values used:
https://github.com/openbmc/phosphor-nvme/blob/master/nvme_manager.cpp#L400

This code is still using integers (uint8_t??) instead of double, so I don't see a feasible way to update this for you. Can someone provide suggestions (or make the update) to take care of the changes required by the above phosphor-dbus-interfaces commit?