Locator dependency being created with invalid version value if it is not specified

Given the following configuration the plugin adds a dependency which `dep.getVersion()` returns `"null"` (a string with `null` value in it):

```groovy
protobuf {
protoc {
artifact = 'com.google.protobuf:protoc'
}
}
```

Before `0.9.6` it was correctly set to `null`. The issue introduced in https://github.com/google/protobuf-gradle-plugin/pull/786.

Whilst everything it seems working fine, it doesn't feel like the correct behaviour and some plugins that operate with dependencies might break/behave incorrectly because of that.