If the accountId is null, it results in a NullPointerException when
attempting to get the account from the account cache:
at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:782)
at com.google.common.cache.LocalCache.get(LocalCache.java:4146)
at com.google.common.cache.LocalCache.getOrLoad(LocalCache.java:4151)
at com.google.common.cache.LocalCache$LocalLoadingCache.get(LocalCache.java:5140)
at com.google.gerrit.server.account.AccountCacheImpl.get(AccountCacheImpl.java:85)
Check for null, and return the name and email address of the server
identity, consistently with what #getNameFor does when the accountId
is null. Update the Javadoc accordingly.
Also update the Javadoc of #getNameFor to make it consistent.
Change-Id: I8145ca79d2122616247b013f83ded50db1f30f62
d75352f042