Android API 23 and below does not like Mac.init with a null key

[05nelsonm]

06-06 15:52:15.751  3156  3175 E TestRunner: java.security.InvalidKeyException: key == null
06-06 15:52:15.751  3156  3175 E TestRunner: 	at javax.crypto.Mac.init(Mac.java:321)
06-06 15:52:15.751  3156  3175 E TestRunner: 	at org.kotlincrypto.core.Mac.<init>(Mac.kt:60)
06-06 15:52:15.751  3156  3175 E TestRunner: 	at org.kotlincrypto.macs.Hmac.<init>(Hmac.kt:57)
06-06 15:52:15.751  3156  3175 E TestRunner: 	at org.kotlincrypto.macs.Hmac.<init>(Hmac.kt:45)
06-06 15:52:15.751  3156  3175 E TestRunner: 	at org.kotlincrypto.macs.HmacSHA3_512.<init>(HmacSHA3_512.kt:36)

Android API 23 and below seem to be affected.

As we're installing our own engine with the engineInit overridden and no-op'd, would be better to send a static key than use null in order to avoid any potential issues. It does absolutely nothing but set Mac.isInitialized to true.