If you enter the URL as https://YOUR_SITE.com/dokuwiki?do=media&ns=NAMESPACE:NEW_NAMESPACE it will create a new media namespace matching the one entered in the URL (after the ns= ), and you can upload your images into it.
For example, on the DokuWiki.org wiki, the namespace mammals:whales: doesn't exist, but if I visited the URL
https://www.dokuwiki.org/dokuwiki?do=media&ns=mammals:whales I could upload images to a new namespace named whales without there having to be any pages in that namespace.