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