I would think that the heater would decrease the humidity in your apartment, not increase it.
That.^
(Assuming it's an electric heater) I wouldn't worry about the heater itself, creating or making worse any issues with humidity. That's not to say humidity is guaranteed to be low enough to be safe for your knives, in using the heater. But the heater alone shouldn't create any greater risk.
If the apartment isn't otherwise environmentally controlled (sealed windows, etc), the humidity could still be an issue, depending on how high it might normally be in your location. If the relative humidity indoors is ~ 40% - 50% or higher, there may still be some risk for rusting. Probably don't have much to worry about though, if the relative humdity is 30% or lower, indoors.
I lived in Texas for 20 years, and the typical humidity indoors at my house was usually somewhere between 40% - 60%, depending on the season and whether I was running the A/C (which pulls moisture from the air and therefore lowers the humidity indoors). That was a little bit marginal for risk of rusting with some of my knives, which could develop some spots in those conditions. Since then, I've moved back to my home state of New Mexico, where it's typically much, much drier. Indoor humidity levels are rarely above 30% - 40% at all, and most of the time, much lower than that. Almost no risk for rusting, for my knives, in that environment. That's my frame of reference, in estimating what's probably 'safe' for my knives.
