Front axle/spindle bolt broke

No., it's not safe to ride on. If the bolt wound in easily, if you take the weight of the wheel off the spindle, you can probably unwind it with your fingers, otherwise this is one of the recommended scenarios for an easy-out. would try and turn it with a scribe or punch first though, without the head, there is no clamping force, so the threads aren't doing any holding. I have also used a flat head and ground out the center of the flat so it's 2 prongs, it digs in to the broken bolt surface and you can just undo.

What year, brand. and model bike is this? Is this the front or rear wheel? I have never seen a spindle with a hex bolt style head.
And agree with Richard - NOT Safe.