#701 - DB Migration - incorrect alter column DDL when removing not null constraint (Postgres specific)

src/main/java/com/avaje/ebean/dbmigration/ddlgeneration/platform/PostgresDdl.java

@@ -13,6 +13,7 @@ public class PostgresDdl extends PlatformDdl {
     this.dropTableCascade = " cascade";
     this.columnSetType = "type ";
     this.alterTableIfExists = "if exists ";
+    this.columnSetNull = "drop not null";
   }
 
   /**