Fix non-empty dir warning message when setting default EC policy

When EC policy is set on a non-empty directory, the following warning message is given:

$hdfs ec -setPolicy -policy RS-6-3-1024k -path /ec1
Warning: setting erasure coding policy on a non-empty directory will not automatically convert existing files to RS-6-3-1024k

When we do not specify the -policy parameter when setting EC policy on a directory, it takes the default EC policy. Setting default EC policy in this way on a non-empty directory gives the following warning message:

$hdfs ec -setPolicy -path /ec2
Warning: setting erasure coding policy on a non-empty directory will not automatically convert existing files to null

Notice that the warning message in the 2nd case has the ecPolicy name shown as null. We should instead give the default EC policy name in this message.