Bugfix: th and td elements can be empty

[Convincible]

It is possible and valid to have an empty <td></td> element (similarly with <th>) and would be invalid to close it as <td />.

This change adds th and td to the list of elements that must be explicitly closed even if they have no content.

It avoids a bug where the invalid output <td /> will be generated if a table cell has no content.

[gettalong]

Thanks for your contribution! Could you please add a test case in test/testcases/block/14_table?

[gettalong]

Thanks for opening this issue - I have fixed this for the next release.