Fix compilation error when BuildInfo includes git tag info

Whoopsy-daisy, the project doesn't compile if git HEAD is a tag!

```
[error]  found   : Seq[String]
[error]  required: Seq[play.api.libs.json.JsValue]
[error]       "git_tags" -> JsArray(BuildInfo.gitTags),
```

Unless you're on a tag, the auto-generated `BuildInfo` has an empty tag list with no type declaration, so it compiles fine.