[BUGFIX] Fix invalid default values in sg_mail

ext_tables.sql

-#
-# Table structure for table 'tx_sgmail_domain_model_mail'
-#
 CREATE TABLE tx_sgmail_domain_model_mail (
 	uid int(11) NOT NULL auto_increment,
 	pid int(11) DEFAULT '0' NOT NULL,
@@ -25,20 +22,16 @@ CREATE TABLE tx_sgmail_domain_model_mail (
 
 	PRIMARY KEY (uid),
 	KEY parent (pid)
-
 );
 
-#
-# Table structure for table 'tx_sgmail_domain_model_template'
-#
 CREATE TABLE tx_sgmail_domain_model_template (
 	uid int(11) NOT NULL auto_increment,
 	pid int(11) DEFAULT '0' NOT NULL,
 
 	subject text NOT NULL,
-	extension_key varchar(255) DEFAULT NULL NOT NULL,
-	template_name varchar(255) DEFAULT NULL NOT NULL,
-	language varchar(30) DEFAULT NULL NOT NULL,
+	extension_key varchar(255) DEFAULT '' NOT NULL,
+	template_name varchar(255) DEFAULT '' NOT NULL,
+	language varchar(30) DEFAULT '' NOT NULL,
 	content text NOT NULL,
 
 	tstamp int(11) unsigned DEFAULT '0' NOT NULL,
@@ -51,5 +44,4 @@ CREATE TABLE tx_sgmail_domain_model_template (
 
 	PRIMARY KEY (uid),
 	KEY parent (pid)
-
 );