Mosfet drain/source minimum voltage

[speedy_rudolf]

Hi, everyone.
I have a very basic question. I have a couple mosfets in half-bridge configuration. I would like to know what could happen if I turned the mosfets on, but there was no current on either drain or source pins.
A basic circuit representation is as in the image attached. So, basically, both h-bridges are turned on by the same driver, but only one of them gets any voltage at all. What would happen to the mosfets when the opto-triac is switched off and the mosfet switched on?
Thanks in advance.

[PSR B1257]

what could happen if I turned the mosfets on

You can't turn them on in this configuration.
Just applying a potential on the gate of the MOSFET does not turn it on. That requires a potential difference from gate to source. In your case it requires more specifically floating voltages for each MOSFET.
What are the opto-triacs for (in your mind)?

[speedy_rudolf]

Well, the optos are there to "apply" voltage only to specific group of mosfets/h-bridges. I know, i could have used other components as wel (mosfets for one), but i chose opto-triacs because of reverse voltage spikes (the h-bridges will eventually be used for switching inductive loads).
but back to my main question. Let me rephrase: will it be a problem (maybe destroy the mosfets) if i apply voltage to the gate, but there is no voltage/current on either drain and source pins?

[PSR B1257]

the optos are there to "apply" voltage only to specific group of mosfets/h-bridges. I know, i could have used other components as wel (mosfets for one)

I dont see the point. You already have two switches. If you have no driving voltage, the MOSFETs will block any current anyway.
A H-bridge consists of two sides. Just applying voltage to one side is meaningless since there is no current flow at the other side.
The bipolar supply voltage is also not necessary. The bridge applies the total supply voltage (in your case 440V) to the load. It does not matter whether it is a uni- or bipolar supply voltage.

of reverse voltage spikes (the h-bridges will eventually be used for switching inductive loads).

What should the triacs do about these voltage spikes, ha 
You need to put free wheling diodes aross the MOSFETs in order to clamp these voltages.

will it be a problem (maybe destroy the mosfets) if i apply voltage to the gate, but there is no voltage/current on either drain and source pins?

No.