plan: fix error when variable is not set #2755

Job	Run time
Check secrets / Check secret	2s
Packaging (xenial)	4m 20s
Packaging (bionic)	6m 26s
Packaging (focal)	3m 49s
Packaging (jammy)	4m 7s
Packaging (lunar)	3m 58s
Packaging (mantic)	2m 57s
Integration (bionic, lxd, ubuntu-22.04)	1h 8m 45s
Integration (focal, lxd, ubuntu-22.04)	1h 15m 2s
Integration (jammy, lxd, ubuntu-22.04)	56m 32s
Integration (lunar, lxd, ubuntu-22.04)	31m 5s
Integration (xenial, lxd, ubuntu-20.04)	1h 11m 31s
Integration (bionic, awspro, ubuntu-22.04)	38m 28s
Integration (bionic, gcppro, ubuntu-22.04)	43m 32s
Integration (bionic, awspro-fips, ubuntu-22.04)	16m 22s
	7h 6m 56s

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