Fix unwanted curly braces in formula (bug #8679)

Fix unwanted curly braces in formula (bug #8679)

There was an unsymmetry between reading and writing: InsetMathGrid::eolString()
adds curly braces if the first cell of the next line starts with [ to prevent
misparsing as optional argument of \\. These braces were not removed on reading.

Thanks to Enrico, who noticed that the previous fix did not take into account
the case of nonempty length argument + the next line beginning with [.
Now the parsing is exactly the inverse of InsetMathGrid::eolString().