Properly handle empty TableName in TablePermission#readFields and #write

HBASE-17450 handles the empty table name in equals().

This JIRA is to properly handle empty TableName in TablePermission#readFields() and TablePermission#write() methods and also null checks whereever appropriate in the code to avoid NPE. The toString method if-elseif-else condition check causes some of the details not to be logged for a given TablePermission so this has to be fixed