LaTeX: complex table with merged cells broken since 4.0

[jfbu]

Describe the bug
A clear and concise description of what the bug is.

! Argument of \TY@get@body has an extra }.
<inserted text> 
                \par 
l.107 ...\multicolumn{2}{|l|}\sphinxtablestrut{1}}
                                                  &

To Reproduce
Steps to reproduce the behavior:

TITLE
#####

Complex table
=============

+--------------------+----------------+
|  2 rows and 2 cols | 1 row x 3 cols |
|                    +----+-----+-----+
|                    |  A |  B  |  C  |
+---+----------------+----+-----+-----+
| 1 |       2        |  3 |  4  |  5  |
+---+----------------+----+-----+-----+

and make latexpdf

Expected behavior

make latexpdf succeeds

Environment info

• OS: [e.g. Unix/Linux/Mac/Win/other with version]
• Python version: 3.8.7
• Sphinx version: since 4.0
• Sphinx extensions: [e.g. sphinx.ext.autodoc, recommonmark]
• Extra tools: TeXLive 2019

Additional context
First bad commit is 20884bb (#8875)

There seems to be an extra } in latex output in tables. Sorry for not spotting problem.

• [e.g. URL or Ticket]