It should not be possible to release an invalid schedule

Current Situation

Workflow W1 is a workflow. Variable V1 is a workflow variable of W1. V1 has no default. W1 is assigned to Schedule S1.
S1 is invalid as long as V1 has no value.
After specifying a value for V1, S1 becomes valid.
If the value is now deleted, S1 can be released.

Desired Behavior

It should not be possible to release an invalid schedule.