Pipeline options validation

[Ning Wang]

Hi, Beam devs,

I am working on a runner and found something not working as expected.

I have this field in my H*PipelineOptions,
```
  @Description("Number of Containers")
  @Validation.Required
  int getNumberOfContainers();
  void setNumberOfContainers(int value);
```
and I am calling this validation function,
```
H*PipelineOptions options =
    PipelineOptionsValidator.validate(H*PipelineOptions.class, opts);
```

I am expecting that if --numberOfContainer is missing in command line,
there should be an error, however it seems like the value is set to 0 by
default.

Is this the expected behavior? Or is there anything missing? My Beam
version is 2.11.0.

Thanks in advance!
--ning