If output_cmd_background is given a valid mode as the first argument,
then there is no file given and an error should be returned.
join_args should not be called with an argc of zero since it sets the
last character to the null terminator. With an argc of zero, the length
is zero causing a heap buffer overflow when setting the byte before the
start of argv to '\0'. This probably will not ever generate a segfault,
but may cause data corruption to whatever is directly before it in
memory. To make other such cases easier to detect, this also adds a
sway_assert in join_args when argc is zero.
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