Fix backward compatibility of config file...

Venus 1.1.3 crashes on startup when using certain config files that work ok with 1.1.1.

I think the code was assuming that the config always contains:
```
{
   libraries: {includes: []}
}
```

whereas my config contained:
```
{
   libraries: {}
}
```
Keeping a simple default empty config seems desirable: this commit makes a tiny change to be ok when the nested `includes` is missing.