According to the documentation,
An array variable is considered set if a subscript has been assigned a value. The null string is a valid value.
No subscript has been assigned a value, so the array isn’t set.
But while the documentation suggests an error is appropriate here, this is no longer the case since 4.4.
$ bash --version | head -n 1
GNU bash, version 4.4.19(1)-release (x86_64-pc-linux-gnu)
$ set -u
$ arr=()
$ echo "foo: '${arr[@]}'"
foo: ''
There is a conditional you can use inline to achieve what you want in older versions: Use ${arr[@]+"${arr[@]}"} instead of "${arr[@]}".
$ function args { perl -E'say 0+@ARGV; say "$_: $ARGV[$_]" for 0..$#ARGV' -- "$@" ; }
$ set -u
$ arr=()
$ args "${arr[@]}"
-bash: arr[@]: unbound variable
$ args ${arr[@]+"${arr[@]}"}
0
$ arr=("")
$ args ${arr[@]+"${arr[@]}"}
1
0:
$ arr=(a b c)
$ args ${arr[@]+"${arr[@]}"}
3
0: a
1: b
2: c
Tested with bash 4.2.25 and 4.3.11.