1.1.3 had a bug where task.VolumeMounts will be an empty slice instead of nil. Eventually, it gets canonicalized and is set to `nil`, but it seems to confuse dry-run planning.

The regression was introduced in https://github.com/hashicorp/nomad/pull/10855/files#diff-56b3c82fcbc857f8fb93a903f1610f6e6859b3610a4eddf92bad9ea27fdc85ecL1028-R1037 . Curiously, it's the only place where `len(apiTask.VolumeMounts)` check was dropped. I assume it was dropped accidentally.

Fixes #10981