`Strategy.invoke` accepts wrong argument set

This works correctly.

```ruby
Trailblazer::Pro::Generate.invoke([{json_document: collaboration_json}, {}])
```

This "works", but breaks in VariableMapping .

```ruby
Trailblazer::Pro::Generate.invoke([{json_document: collaboration_json}])
```

The latter version should either default `flow_options` to an empty hash, or break right away.