Change CI_DB_driver::field_data() signature

The parameter is mandatory, it doesn't make sense to have a default
empty string value only to check for it.
diff --git a/system/database/drivers/mssql/mssql_driver.php b/system/database/drivers/mssql/mssql_driver.php
index 2fd21db..1aeff1d 100644
--- a/system/database/drivers/mssql/mssql_driver.php
+++ b/system/database/drivers/mssql/mssql_driver.php
@@ -344,13 +344,8 @@
 	 * @param	string	$table
 	 * @return	array
 	 */
-	public function field_data($table = '')
+	public function field_data($table)
 	{
-		if ($table === '')
-		{
-			return ($this->db_debug) ? $this->display_error('db_field_param_missing') : FALSE;
-		}
-
 		$sql = 'SELECT COLUMN_NAME, DATA_TYPE, CHARACTER_MAXIMUM_LENGTH, NUMERIC_PRECISION, COLUMN_DEFAULT
 			FROM INFORMATION_SCHEMA.Columns
 			WHERE UPPER(TABLE_NAME) = '.$this->escape(strtoupper($table));