Update CDbCommand.php

The from function will ignore any empty table definition when $tables are passed as an array

ex before fix: 
$table = someLogic(); // return the table sql from somelogic
createCommand(array('from'=>array('table1', $table))); // $table is null or empty string in this case
The SQL generated will be somthing like this => SELECT * FROM table1,

after fix: SELECT * FROM table1