Don't require project ids to be non-empty in XtbMessageBundle
The parameter is documented to be nullable and the GoogleJsMessageIdGenerator supports that.  Also, the SSTableMessageBundle doesn't appear to require this.
@@ -85,7 +85,6 @@ public final class XtbMessageBundle implements MessageBundle {
* @param projectId the translation console project id (i.e. name)
*/
public XtbMessageBundle(InputStream xtb, @Nullable String projectId) {
- checkState(!"".equals(projectId));
this.messages = new HashMap<>();
this.idGenerator = new GoogleJsMessageIdGenerator(projectId);
