Support set but empty env variables

[jklap]

Act version

act version: 0.2.75

Feature description

There are scenarios where we want to support sending empty vars and/or secrets into a workflow. But attempting to do this via any of the below scenarios doesn't work:

export MY_VAR=''
act --var MY_VAR
# act asks for input
export MY_SECRET=''
act --secret MY_SECRET
# act asks for input
act --var MY_VAR=''
# act asks for input
act --secret MY_SECRET=''
# act asks for input

We should support the ability to pass in a var or secret via the command line or a set-but-empty env var