Bash promises to leave incorrectly-formed brace expressions unchanged, and
this is why {} on input yields {} on find's command-line. {} is
incorrectly formed since it doesn't contain a sequence expression or a
comma. But is the same guarantee provided by other shells that also
perform brace expansion?
zsh's brace expansion also produces {} for the input {} but the
documentation could be read as if the empty string were also a valid result
when the BRACE_CCL option is set.
