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You're getting the error because you haven't defined a != operator for a HashTable (which is what your message says, right?). table [position] is a pointer, but if you dereference it, you're referring to a HashTable. You should test the pointer for NULL before you dereference it.
As for NULL, it's implementation defined. There's a good chance it's zero. Every location in memory has something in it. It's a piece of hardware that has bit cells that can only contain the states, 0 and 1 (except for a short period of time when it's transitioning between the two). There is no "nothing"; there's just an agreement that some "something" will represent "nothing".
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