Enable null globs when scanning for config.d/*.sh

[nogweii]

Without this, an empty directory leads to an false error complaining that the file /etc/dehydrated/config.d/*.sh is not readable or doesn't exist.

With nullglob set, the glob, if it doesn't match anything, will expand into null rather than returning the glob string, causing an empty config.d folder to not throw an error.

[silkeh]

Thanks for this PR, seems sane to me generally. However, the unit tests fail because the script now exits with a zero exit code.
You can see this locally:

$ env VERBOSE=yes ./test.sh tests/hooks.sh
Testing using Bash 5.1.16(1)-release
Hooks
 + exit 	[PASS]
 + deploy 	[PASS]
Hook errors
 + exit 	[PASS]
 + deploy 	[PASS]
Hook invalid scripts
 + exit 	[FAIL]
EXPECTED
 ./pdns_api.sh: line 369: /Foo3Xeiz: No such file or directory exit=127
 GOT
 ./pdns_api.sh: line 373: /Foo3Xeiz: No such file or directory
exit=127
 ---
 + deploy 	[FAIL]
EXPECTED
 ./pdns_api.sh: line 375: /dah5Aiph: No such file or directory exit=127
 GOT
 ./pdns_api.sh: line 379: /dah5Aiph: No such file or directory
exit=127
 ---
Tests complete: 2 failures.

[silkeh]

See comment above.