Merge of change https://widevine-internal-review.googlesource.com/7950. In the OEMCrypto library, it is valid to call LoadKeys with an empty mac key. The library should not update the mac and enc keys and should not throw an error. Since we have seen this behavior in several implementations, a unit test should be added that verifies the correct behavior. bug: 11032674 Change-Id: I011ba39c8abc47547226a722143e92dac3f63dc5