bundle2: handle empty 'b2x:changegroup' value in push and pull Changeset e4dc2b0be056 added advertising of supported changegroup version through the new 'b2x:changegroup' capability. However, this capability is not new and has been around since 3.1 with an empty value. This makes new clients unable to push to 3.2 servers through bundle2 as they cannot find a common changegroup version to use from and empty list. Treating empty 'b2x:changegroup' value as old client fixes it.