After installing 10.1 on my German system, I was unable to change the trash warning prefs (it would change but not stick).
I have discovered that it seems to be a problem of the German localization of 10.1. The GUI workaround that I found is to switch to English as a system language and uncheck the trash warning box. When you change back your original language setting, your Finder settings will stick.
A terminal session with the line "defaults write com.apple.finder WarnOnEmptyTrash false" should do the same (although I did not test!).
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