Custom dynamicDefaults functions don't work with default parameters and no args given in schema

The examples of how to add a custom uuid function dynamicDefaults wrap `uuid.v4()`. However, I'd expect to be able to use either just `uuid` (which calls `uuid.v4`) or `uuid.v4` itself, without a `function uuidv4()` wrapper.

It seems that the code in [dynamicDefaults](https://github.com/ajv-validator/ajv-keywords/blob/master/keywords/dynamicDefaults.js#L30) uses `func.length figure out if the value of the custom dynamicDefault should be called with arguments.

```javascript
funcs[key] = func.length ? func(d.args) : func;
```

(I didn't know this, but In javascript, [Function.length is the number of arguments](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/length).)

```javascript
> uuid = require('uuid');
{ [Function: v4] v1: [Function: v1], v4: [Circular] }
> uuid.length
3
```

I guess `func.length` is being used to figure out if arguments should be passed to the func as defined in the schema via `{func: "uuid", args }`, but perhaps a better way would be to just check if `args` is defined?

It'd be nice if I didn't have to make a wrapper for functions that have default parameters.