face-detect-service input no-face image without Exception

**Is your feature request related to a problem? Please describe.**
why face-detect-service will return exception msg if input no-face image

**Describe the solution you'd like**
return the same struct with empty list of result if image has no face.

```
{
"result" : [],
"plugins_versions" : {
"age" : "agegender.AgeDetector",
"gender" : "agegender.GenderDetector",
"detector" : "facenet.FaceDetector",
"calculator" : "facenet.Calculator",
"mask": "facemask.MaskDetector"
}
}
```

**Describe alternatives you've considered**
other rest api should also consider this problem

**Additional context**