isV6Format returns "true" when passed a blank string.

Hi, when I pass a blank string to `isv6Format()` it returns `true`, but a blank string is not a valid IPv6 address.

```javascript
const ip = require("ip");
ip.isV6Format(""); // true
```
Is this intended behaviour?

Thanks,

Start at the isV6Format() entry point and reproduce its behavior with a blank string. Confirm the intended result for blank input, then make the behavior consistent with whether the input is a valid IPv6 address and verify that the reported example no longer returns true.

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