I think it used to be a nickname for either Helen or Eleanor… but over time, it became a name in its own right. Some of the more famous Nells, like Nell Cater, have “Nell” as their given names.
The most famous Nellies I can think of, however, were men. MVP second baseman Nelson “Nellie” Fox, and rapper Cornell “Nelly” Haynes.
Around here, Nel and Nellie are usually short for Petronella, which is I believe the feminized version of Peter, or for Cornelia. However, this is almost certainly not true where you are. In English I thought it was the short form of Helen, but apparently not.
For guys, it’s a variation on Nelson as for Nellie Fox, Baseball Hall of Famer. My father also had some friends named Nelson back in the '20s and '30s he called Nellie.