FIX: Splitcolumns must return an empty array if string was empty (#1315)

* Add test suite from rpraml

* FIX: Splitcolumns must return an empty array if string was empty

* Just the reference scripts

src/main/java/io/ebeaninternal/dbmigration/ddlgeneration/platform/SplitColumns.java

@@ -6,6 +6,9 @@ public class SplitColumns {
    * Return as an array of string column names.
    */
   public static String[] split(String columns) {
+    if (columns == null || columns.isEmpty()) {
+      return new String[0];
+    }
     return columns.split(",");
   }