VolumeBuilder does not allow null id while VolumeImpl allows it to be nullable

I have been busy serializing Nodemetadata and all other Jclouds constructs and found that VolumeBuilder does a checkNotNull on id. However if .id("id") is not called it remains null for the VolumeImpl that is created when building.

It is a small issue, and the fix is to remove CheckNotNull at VolumeBuilder, which should not break anything.

While this is a really small bug, it means that during building of a volume, the user has to do a separate call and check if id is null before calling VolumeBuilder. The other Jclouds constructs Image/Hardware/NodeMetadata/Location/OperatingSystem can all be created in a single xxBuilder call.

Details:
See VolumeBuilder being used here without id with it not being supplied in many cases:
https://github.com/jclouds/jclouds/blob/4c74b497547e42b8bdc94dbae3d4cd94ff3945d6/apis/cloudservers/src/test/java/org/jclouds/cloudservers/compute/functions/ServerToNodeMetadataTest.java

I
https://github.com/jclouds/jclouds/blob/4c74b497547e42b8bdc94dbae3d4cd94ff3945d6/compute/src/main/java/org/jclouds/compute/domain/VolumeBuilder.java

Then see VolumeImpl allowing a Nullable id:
https://github.com/jclouds/jclouds/blob/4c74b497547e42b8bdc94dbae3d4cd94ff3945d6/compute/src/main/java/org/jclouds/compute/domain/internal/VolumeImpl.java