Scifi YA story about boy leaving safe surroundings for supposedly unsafe environment

Question

• I read this story in German in the early to mid 1990's. As it was in the local library and looked quite used it might be older than that.
• The cover I remember was a figure shown from behind in front of a Mars-like environment (red wasteland, black sky)
• I distinctly remember the book title to be "Keiner" (German for nobody).
• The Story as I remember was that everybody lived in an underground facility because the above world was a barren wasteland. One boy decided to find out and travelled out. He discovered that indeed the above ground was barren, but not as deadly as proposed. Somehow I remember there being reptilian aliens in it, who took him in. As they asked him his name he said he was nobody (hence the title "Keiner" or nobody).