Nightvision - Fix script error when switching to thermals

[PabstMirror]

_fogApply = linearConversion [0, 1, ace_nightvision_>
 Error position: <linearConversion [0, 1, ace_nightvision_>
 Error Type Any, expected Number
File z\ace\addons\nightvision\functions\fnc_pfeh.sqf..., line 165

fix script error when going directly to thermals
e.g. the MX binoc from ace or some mods that have vics with just thermals (no nvg first)

also skips all other effects if just in thermal