Nedra is a name inspired from a novel, namely 'Nedra' by George Barr McCutcheon published in 1905. In the novel, Nedra is the name of an island where the protagonists are stranded, but the cover of the novel can give the impression that it were a girl's name. After that, Nedra was reused for several characters in early films. As far as I know, Nedra has no meaning (neither in-story nor externally).
You can find a good post about this on Nancy Man's site here:
http://www.nancy.cc/2017/04/19/mccutcheon-baby-names-yetive-nedra/