clipboard_paste: Require non-empty clipboard contents

Fixes #3637.

share/functions/fish_clipboard_paste.fish

@@ -5,7 +5,9 @@ function fish_clipboard_paste
         # Only run `commandline` if `xsel` succeeded.
         # That way any xsel error is printed (to show e.g. a non-functioning X connection),
         # but we don't print the redundant (and overly verbose for this) commandline error.
+        # Also require non-empty contents to not clear the buffer.
         if set -l data (xsel --clipboard)
+            and test -n "$data"
             commandline -i -- $data
         end
     end