create_simulation_folder() does not check for empty strings

If you pass an empty string to `basedir` or `simdir` in `create_simulation_folder()`, hijinks ensue. Some of the hijinks may be permissible, some we should definitely avoid. For fun, compare:

```R
create_simulation_folder(basedir = "", simdir = "")
```
and

```R
create_simulation_folder(basedir = "", simdir = NULL)
```
