blobInfo.toBuilder().build() is not equal to blobInfo

We have this situation:

``` java
BlobInfo blobInfo = BlobInfo.builder(BlobId.of("b", "n")).build();
assertEquals(blobInfo, blobInfo.toBuilder().build()); // This fails!
```

This is due to the fact that `toBuilder` uses builder's setters that initialize (some of them) fields to `Data.nullOf()` if `null` is passed. This can be fixed by checking for `null` before calling setters in `toBuilder()`.
