Remove line in text export of inline math (#9282)

If there is only one row, a new line should not be appended. If
there is more than one row, the new line should not be appended for
the last row.

For example, if in LyX there was "as you can see in $x$ there..." a
copy/paste would lead to a newline after "x".
No space after inline math for text export (#9282)

This extra space was the result of the fix for #6094
(at 7e624928).