Fix check-docs.sh failing on macOS when an option has no values

[jakepetroules]

The macOS documentation check always fails immediately:

check-docs.sh: line 68: collected[*]: unbound variable

soundness.yml always passes --additional-docc-arguments ${ADDITIONAL_DOCC_ARGUMENTS}, so when no additional arguments are configured the option is passed with no values and collected stays empty. macOS runners use /bin/bash 3.2, where expanding an empty array under set -u is treated as an unbound variable (bash 4.4+ allows it), so the script exits before building any documentation. The Linux docs check isn't affected because its container has a newer bash.

--doc-targets has the same problem, so this changes both expansions to "${collected[*]:-}".

Tested locally with macOS /bin/bash 3.2.57 against a package:

• check-docs.sh --additional-docc-arguments (the default CI invocation): previously failed with the error above, now passes
• check-docs.sh --no-analyze --doc-targets --additional-docc-arguments: passes
• check-docs.sh --doc-targets <target> --additional-docc-arguments --include-extended-types: values are still forwarded to generate-documentation, passes