Why there is Cp/Ct (crossing point) value for negative sample in COVID-19 RT-PCR?
PCR is highly specific (meaning it amplifies only the DNA the primers bind to), so the results in general are good. But: Due to the exponential nature of amplification, even very small mispairings can lead to the amplification of a very small amount of DNA and subsequently to a fluorescent signal. There are basically two ways of getting this amplification (except from the desired one): Contaminations and primer self-amplification. The first one should be clear, the second one happens when primers are able to bind to themself (or the complimentary one). This usually shows up late in the PCR, which is the reason why you assign a Ct-value to it after which you do not expect any meaningful amplification result anymore.

Depending on the method, you extract all RNA from your sample and translate it into cDNA. However, for the amplification in the PCR you need primers that fit to this cDNA. And since these primers have been proven to be highly specific (meaning: they bind absolutely no other sequence, not even from closely related viral species) we can be sure that any signal which is about the criteria mentioned in 1. comes from viral RNA and shows an infection. See the reference for one of the often used PCR tests.

Without seeing the curve, I cannot say anything specific about it. In general a real-time PCR curve follows a sigmoid shape. This gives some information about the way the reaction progressed over time. If it looks differently, something may have either happened to the reaction or to the sample. It may contain a PCR inhibitor, the sample can be contaminated or something else. I wouldn't trust such a result and repeat it with a new sample.


Reference
Detection of 2019 novel coronavirus (2019-nCoV) by real-time RT-PCR