org-latex-line-break-safe: Use safer value of "\\[0pt]"

* lisp/ox-latex.el (org-latex-line-break-safe):
(org-latex-table-row):
Change \empty ending to explicit optional argument.  \empty still has
undesired side effects in some cases.

* testing/lisp/test-org-table.el (test-org-table/to-latex):
* testing/lisp/test-ox-latex.el (test-ox-latex/verse): Update tests.

Reported-by: Juan Manuel Macías <maciaschain@posteo.net>
Link: https://orgmode.org/list/87o7u9rz1a.fsf@posteo.net

lisp/ox-latex.el

@@ -278,17 +278,17 @@ list can be:
 
 - `:lang-name' the actual name of the language.")
 
-(defconst org-latex-line-break-safe "\\\\\\empty"
+(defconst org-latex-line-break-safe "\\\\[0pt]"
   "Linebreak protecting the following [...].
 
-Without \"\\empty\" it would be interpreted as an optional argument to
+Without \"[0pt]\" it would be interpreted as an optional argument to
 the \\\\.
 
 This constant, for example, makes the below code not err:
 
 \\begin{tabular}{c|c}
-    [t] & s\\\\\\empty
-    [I] & A\\\\\\empty
+    [t] & s\\\\[0pt]
+    [I] & A\\\\[0pt]
     [m] & kg
 \\end{tabular}")
 
@@ -4005,9 +4005,9 @@ a communication channel."
 			      (org-export-get-parent-table table-row) info))))
 	   (format "%s
 \\endfirsthead
-\\multicolumn{%d}{l}{%s} \\\\\\empty
+\\multicolumn{%d}{l}{%s} \\\\[0pt]
 %s
-%s \\\\\\empty\n
+%s \\\\[0pt]\n
 %s
 \\endhead
 %s\\multicolumn{%d}{r}{%s} \\\\

testing/lisp/test-org-table.el

@@ -1635,11 +1635,11 @@ See also `test-org-table/copy-field'."
 (ert-deftest test-org-table/to-latex ()
   "Test `orgtbl-to-latex' specifications."
   (should
-   (equal "\\begin{tabular}{l}\na\\\\\\empty\n\\end{tabular}"
+   (equal "\\begin{tabular}{l}\na\\\\[0pt]\n\\end{tabular}"
 	  (orgtbl-to-latex (org-table-to-lisp "| a |") nil)))
   ;; Test :environment parameter.
   (should
-   (equal "\\begin{tabularx}{l}\na\\\\\\empty\n\\end{tabularx}"
+   (equal "\\begin{tabularx}{l}\na\\\\[0pt]\n\\end{tabularx}"
 	  (orgtbl-to-latex (org-table-to-lisp "| a |")
 			   '(:environment "tabularx"))))
   ;; Test :booktabs parameter.
@@ -1648,7 +1648,7 @@ See also `test-org-table/copy-field'."
     "\\toprule" (orgtbl-to-latex (org-table-to-lisp "| a |") '(:booktabs t))))
   ;; Handle LaTeX snippets.
   (should
-   (equal "\\begin{tabular}{l}\n\\(x\\)\\\\\\empty\n\\end{tabular}"
+   (equal "\\begin{tabular}{l}\n\\(x\\)\\\\[0pt]\n\\end{tabular}"
 	  (orgtbl-to-latex (org-table-to-lisp "| $x$ |") nil)))
   ;; Test pseudo objects and :raw parameter.
   (should

testing/lisp/test-ox-latex.el

@@ -60,14 +60,14 @@ lorem ipsum dolor
     (should
      (search-forward
       "\\begin{verse}
-lorem ipsum dolor\\\\\\empty
-lorem ipsum dolor\\\\\\empty
+lorem ipsum dolor\\\\[0pt]
+lorem ipsum dolor\\\\[0pt]
 \\vspace*{1em}
-lorem ipsum dolor\\\\\\empty
-lorem ipsum dolor\\\\\\empty
+lorem ipsum dolor\\\\[0pt]
+lorem ipsum dolor\\\\[0pt]
 \\vspace*{1em}
-lorem ipsum dolor\\\\\\empty
-lorem ipsum dolor\\\\\\empty
+lorem ipsum dolor\\\\[0pt]
+lorem ipsum dolor\\\\[0pt]
 \\end{verse}"))))
 
 (provide 'test-ox-latex)