Update preconditions in AbstractUfsManager.java

[gpang]

In /core/server/common/src/main/java/alluxio/underfs/AbstractUfsManager.java, in method addMount(long mountId, final AlluxioURI ufsUri, final UnderFileSystemConfiguration ufsConf), change the preconditions:

Preconditions.checkArgument(ufsUri != null, "ufsUri");
Preconditions.checkArgument(ufsConf != null, "ufsConf");

to

Preconditions.checkNotNull(ufsUri, "ufsUri");
Preconditions.checkNotNull(ufsConf, "ufsConf");