cat includes/definitions/discovery/macosx.yaml 
mib: UCD-SNMP-MIB
modules:
    sensors:
        delay:
            data:
                -
                    oid: enterprises.ucdavis.extTable.extEntry.extOutput
                    num_oid: .1.3.6.1.4.1.2021.8.1.101.
                    descr: "DownloadTest(s)"
                    index: 1

cat includes/definitions/macosx.yaml 
os: macosx
text: 'Apple OS X'
type: server
icon: apple
over:
    - { graph: device_bits, text: 'Traffic' }
    - { graph: device_processor, text: 'CPU Usage' }
    - { graph: device_mempool, text: 'Memory Usage' }
discovery_modules:
    sensors: 1
poller_modules:
    sensors: 1
discovery:
    - sysObjectID: .1.3.6.1.4.1.8072.3.2.16
    - sysDescr:
        - 'Darwin Kernel Version'

./discovery.php -h test.hostname -v
[snip]
Delay: Dynamic Discovery (delay): Array
(
    [data] => Array
        (
            [0] => Array
                (
                    [oid] => enterprises.ucdavis.extTable.extEntry.extOutput
                    [num_oid] => .1.3.6.1.4.1.2021.8.1.101.
                    [descr] => DownloadTest(s)
                    [index] => 1
                )

        )

)
Data enterprises.ucdavis.extTable.extEntry.extOutput: Array
(
    [1] => Array
        (
            [extOutput] => 1.122
        )

    [2] => Array
        (
            [extOutput] => web
        )

    [3] => Array
        (
            [extOutput] => Wed May  2 09:51:30 PDT 2018
        )

)
Final sensor value: 
Final sensor value: 
Final sensor value: 

^ why am I getting a NULL value for the final sensor value?