Tasks with no output can't be cached

[adamnovak]

I'm trying to plug Toil into MiniWDL's call caching, so I tried a simple workflow that runs a task with no outputs:

version 1.1

workflow TestWorkflow {
    input {
    }
    call TestTask {
    input:
    }
    output {
    }
}

task TestTask {
    input {
    }
    command <<<
        ls
    >>>
    output {
        Int number = 1
    }
    runtime {
        container: "ubuntu:latest"
        cpu: 1
    }
}

The result of the task can be added to the cache, but CallCache.get() always sees it as a cache miss, because the deserialized {} output JSON is a falsy empty dict.

miniwdl/WDL/runtime/cache.py

Line 106 in 475dd3f

if cache:

Probably the condition should have an is not None.