Do not treat zero-cell rows as valid

[notriddle]

Table rows must have at least one cell. A single pipe, on its own, doesn't count.

The behavior on babelmark is pretty diverse, but this behavior is chosen to align with GFM.

✓ means it treats the zero cell lines with only a pipe as not being valid rows, but otherwise parses as a table.

x means it treats the zero cell lines with only a pipe as a valid row.

? means it does something else.

Implementation ->	first	second	third
pulldown-cmark (after this PR)	✓	✓	✓
github/cmark-gfm	✓	✓	✓
pycmarkgfm	✓	✓	✓
pandoc gfm1	✓	✓	✓
parsedown	x	x	✓
php-markdown-extra	x	x	✓
markdown-it	x	x	✓2
DFM	x	x	x
league/commonmark GFM	x	x	x
s9e/TextFormatter	x	x	x
MD4C	x	x	x
cebe	x	x	x
pandoc	x	x	x
multimarkdown	?3	?3	?3
markdig	?3	?3	?3
maruku	?4	?4	?4

Link to Babelmark 3 test

| Table | Header |
|-------|--------|
| Table | Body   |
|
| Not   | Enough |


| Table | Header |
|-------|--------|
| Table | Body   |
|→
| Not   | Enough |
.
<table><thead><tr><th>Table</th><th>Header</th></thead><tbody>
<tr><td>Table</td><td>Body</td></tr>
</tbody></table>
<p>|
| Not   | Enough |</p>
<table><thead><tr><th>Table</th><th>Header</th></thead><tbody>
<tr><td>Table</td><td>Body</td></tr>
</tbody></table>
<p>|→
| Not   | Enough |</p>

| Table | Header |
|-------|--------|
|
.
<table><thead><tr><th>Table</th><th>Header</th></thead><tbody>
</tbody></table>
<p>|</p>

|
|-------|--------|
| Table | Body   |
.
<p>|
|-------|--------|
| Table | Body   |</p>

Footnotes

1. Not on babelmark 3. I ran it on https://pandoc.org/try/?params=%7B%22text%22%3A%22%7C+Table+%7C+Header+%7C%5Cn%7C-------%7C--------%7C%5Cn%7C+Table+%7C+Body+++%7C%5Cn%7C%5Cn%7C+Not+++%7C+Enough+%7C%5Cn%5Cn%7C+Table+%7C+Header+%7C%5Cn%7C-------%7C--------%7C%5Cn%7C%5Cn%5Cn%7C%5Cn%7C-------%7C--------%7C%5Cn%7C+Table+%7C+Body+++%7C%5Cn%22%2C%22to%22%3A%22html5%22%2C%22from%22%3A%22gfm%22%2C%22standalone%22%3Afalse%2C%22embed-resources%22%3Afalse%2C%22table-of-contents%22%3Afalse%2C%22number-sections%22%3Afalse%2C%22citeproc%22%3Afalse%2C%22html-math-method%22%3A%22plain%22%2C%22wrap%22%3A%22auto%22%2C%22highlight-style%22%3Anull%2C%22files%22%3A%7B%7D%2C%22template%22%3Anull%7D ↩

2. Markdown-it changed some time between the version on their playground
   https://markdown-it.github.io/#md3=%7B%22source%22%3A%22%7C%20Table%20%7C%20Header%20%7C%5Cn%7C-------%7C--------%7C%5Cn%7C%20Table%20%7C%20Body%20%20%20%7C%5Cn%7C%5Cn%7C%20Not%20%20%20%7C%20Enough%20%7C%5Cn%5Cn%7C%20Table%20%7C%20Header%20%7C%5Cn%7C-------%7C--------%7C%5Cn%7C%5Cn%5Cn%7C%5Cn%7C-------%7C--------%7C%5Cn%7C%20Table%20%7C%20Body%20%20%20%7C%5Cn%22%2C%22defaults%22%3A%7B%22html%22%3Afalse%2C%22xhtmlOut%22%3Afalse%2C%22breaks%22%3Afalse%2C%22langPrefix%22%3A%22language-%22%2C%22linkify%22%3Atrue%2C%22typographer%22%3Atrue%2C%22_highlight%22%3Atrue%2C%22_strict%22%3Afalse%2C%22_view%22%3A%22html%22%7D%7D and the version Babelmark 3 uses. ↩

3. Multimarkdown and Markdig don't recognize these at tables at all,
   even though I know they support pipe tables. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

4. Maruku swallows much of the text entirely in these test cases. ↩ ↩² ↩³

[Martin1887]

A pity not being aligned with Pandoc, but GFM is the reference here. Thanks!