Some tables are rendered as invalid CommonMark Markdown 1.2 tables

When rendering tables to markdown/1.2, the following cases produce invalid or wrong Markdown tables:

• Table without header row (e.g. |a|b / |c|d in xwiki/2.1): the output starts with the | — | — | separator line, which isn't a valid Markdown table.
• Header cells at the start of every row (e.g. |=h|a / |=h2|b): all rows are considered as header rows, rows are printed on the same line (| h | a || h2 | b |), followed by the separator and no body.
• Pipe character inside a cell (e.g. |x~|y|z): the pipe isn't escaped so the cell is split in two.
• Empty table: an exception is thrown.

Expected

A valid Markdown table is produced in all cases, preserving the cell content.