Bash parameter transforamtion on empty array triggers unset variable.

[Andrew Neff]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-musl
Compiler: gcc
Compilation CFLAGS: -g -O2 -Wno-parentheses -Wno-format-security
uname output: Linux 28e237a5e16f 5.5.7-200.fc31.x86_64 #1 SMP Fri Feb 28
17:18:37 UTC 2020 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-musl

Bash Version: 5.1
Patch Level: 0
Release Status: alpha

Description:
        I do not know if this is related to bash 5.1 erroneously being
"a little aggressive about skipping over empty strings" mentioned
in "Declaring arrays with empty string in one line is bugged", but using
parameter transformation on an empty array, throws an error if "set -u" is
turned on. This was not how bash 4.4 and 5.0 worked. This bug makes it
impossible to check if an empty variable is an array using parameter
transformation while "set -u" is turned on

Repeat-By:
        # Indirection
        docker run -it --rm bash:5.1-alpha bash -uc 'foo(){ echo "${!1@a}";
}; bar=(); foo bar'
        # Empty array
        docker run -it --rm bash:5.1-alpha bash -uc 'bar=(); echo "${bar@a}"'

        # Declared unset array
        docker run -it --rm bash:5.1-alpha bash -uc 'declare -a bar; echo
"${bar@a}"'
        # Empty associative array
        docker run -it --rm bash:5.1-alpha bash -uc 'declare -A Bar=();
echo "${Bar@a}"'
        # Declared unset associative array
        docker run -it --rm bash:5.1-alpha bash -uc 'declare -A Bar; echo
"${Bar@a}"'

        # I also tested on bash:devel-20200805, with the same results

Fix:
        # All five of the above examples work in bash 4.4 and 5.0, and I
would expect the same behavior in bash 5.1
        # Expected behavior that is already there in 5.1 alpha
        docker run -it --rm bash:5.1-alpha bash -uc 'bar=(); echo
"${bar[@]}' # succeeds, but this is already consistent with bash 4.4 and
5.0. So this is expected to succeed.
        docker run -it --rm bash:5.1-alpha bash -uc 'bar=(); echo
"${bar[@]+set}' # echos nothing, but this is already consistent with bash
3.2 and 5.0. So this is expected behavior
        docker run -it --rm bash:5.1-alpha bash -uc 'bar=(); echo "${bar}'
# fails, but this is already consistent with bash 4.4 and 5.0. So this is
expected to fail, there is no 0th element to the array.
        docker run -it --rm bash:5.1-alpha bash -uc 'bar=(); echo
"${bar+set}' # echos nothing, but this is already consistent with bash 3.2
and 5.0. So this is expected behavior

Thanks