Sorttable complains if the table has no body (and fix)

[RichardNeill]

1. It is possible, when auto-generating tables, to have a table with 1 (or more) header rows, 1 (or more) footer rows, and zero main rows in the table.

<thead>
<tr>...</tr>
</thead>
<tfoot>
<tr>...</tr>
</tfoot>

2. If the table contains an explicit <tbody></tbody> section, then sorttable handles it fine.

3. But in the usual case, where the <tbody> tags are omitted (because they are considered implicit) AND the table has no main rows (i.e. there are no <tr> that come after <thead> and before <tfoot>), then sorttable complains and generates JS errors.

4. The fix is to add the following, in makeSortable, just after the test for table.tHead == null:

//If there is no table body (no <tbody>, nor is it implicit because of a normal <tr>), then give up. Otherwise, we get errors later.
if (table.getElementsByTagName('tbody').length == 0){
    //console.log ("disembodied table.");
    return;
}