Fix Pkg.generate() with default `config`

Calling `Pkg.generate("Foo", "MIT")` on `master` currently fails with a non-obvious diagnostic due to `{}` not being convertible to `Dict`:
```
ERROR: `__generate#245__` has no method matching __generate#245__(::Bool, ::Array{None,1}, ::Array{Any,1}, ::ASCIIString, ::ASCIIString)
```