Drop NOT NULL for username column.

Someone please check MySQL script!

SQL/mysql.initial.sql

@@ -11,8 +11,7 @@ CREATE TABLE IF NOT EXISTS ident_switch
 		int(10) UNSIGNED
 		NOT NULL,
 	username
-		varchar(64)
-		NOT NULL,
+		varchar(64),
 	password
 		varchar(64),
 	host

SQL/mysql/2018071600.sql

@@ -0,0 +1,6 @@
+ALTER TABLE
+	ident_switch
+MODIFY
+	username
+	varchar(64)
+	NULL;

SQL/postgres.initial.sql

@@ -13,8 +13,7 @@ CREATE TABLE ident_switch
 		REFERENCES identities(identity_id) ON DELETE CASCADE ON UPDATE CASCADE
 		UNIQUE,
 	username
-		varchar(64)
-		NOT NULL,
+		varchar(64),
 	password
 		varchar(64),
 	host

SQL/postgres/2018081600.sql

@@ -0,0 +1,5 @@
+ALTER TABLE
+	ident_switch
+ALTER COLUMN
+	username
+DROP NOT NULL;