Technically, your Apple Pencil and iPad should suffer no ill effects if you leave your Pencil connected overnight. However, given that the Pencil charges very quickly, you should ask yourself if really need to leave it connected overnight?
The biggest risk to your iPad and Pencil (1st Gen) when leaving them connected together is the possibility of causing physical damage to the Lightning connectors - particularly to the iPad. Whilst both are relatively robust, replacing a broken Pencil is considerably cheaper than repairing a broken iPad.
The Pencil, when plugged-in to the iPad, presents a +6” lever on the Lightning port. All it takes is one careless knock - and for this reason alone, so as to minimise opportunity for avoidable accidents, you should be mindful of leaving them unnecessarily connected together.
I hope you find this to be helpful.