Depends on your definition of a genocide. Convention on the Prevention and Punishment of the Crime of Genocide which was adopted by the UN General Assembly addresses this.The first draft included political killings but it was subsequently dismissed by many nations who disagreed.So, to answer your question.No, as far as I know, there haven't been any reported genocides that never happened that would be classified as a genocide by Convention on the Prevention and Punishment of the Crime of Genocide's standards.
The key to your question is "reported genocides" which I believe refers to something that has occurred in relatively recent history. In general, when it comes to something like genocide or ethnic cleansing the facts are usually pretty evident so it would be hard to have a reported genocide that did not happen.
However, there are all sorts of folks out there that will claim something was a genocide that wasn't really a genocide. Though I'm not sure someone would really considered that reported. There are also some pretty passionate debate on some of those topics, as well. There are some folks who would claim that the Irish Potato Famine was a genocide, for example.
It's surely not so much reported genocides that didn't happen, as historical wars and massacres that can be seen in retrospect as being now being classed as genocidal. For example, the destruction of the Native Americans. The concept of genocide is relatively recent, and dependent on the mechanisation of warfare. But the thorough destruction of another race may have been 'successful' in other times.
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