Fix a regression in 2.9.12 where serializing empty HTML documents would
not add a terminating newline.

  https://gitlab.gnome.org/GNOME/libxml2/-/issues/266

Taken from upstream:

  https://gitlab.gnome.org/GNOME/libxml2/-/commit/92d9ab4c28842a09ca2b76d3ff2f933e01b6cd6f

diff --git a/HTMLtree.c b/HTMLtree.c
--- a/HTMLtree.c
+++ b/HTMLtree.c
@@ -763,11 +763,15 @@ htmlNodeDumpFormatOutput(xmlOutputBufferPtr buf, xmlDocPtr doc,
             if (((xmlDocPtr) cur)->intSubset != NULL) {
                 htmlDtdDumpOutput(buf, (xmlDocPtr) cur, NULL);
             }
-            /* Always validate cur->parent when descending. */
-            if ((cur->parent == parent) && (cur->children != NULL)) {
-                parent = cur;
-                cur = cur->children;
-                continue;
+            if (cur->children != NULL) {
+                /* Always validate cur->parent when descending. */
+                if (cur->parent == parent) {
+                    parent = cur;
+                    cur = cur->children;
+                    continue;
+                }
+            } else {
+                xmlOutputBufferWriteString(buf, "\n");
             }
             break;
 
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<div class='path'>path: <a href='/guix/log/?id=c93f83757a8fcbeb294e43b43eec0725963005be'>root</a>/<a href='/guix/log/gnu?id=c93f83757a8fcbeb294e43b43eec0725963005be'>gnu</a>/<a href='/guix/log/gnu/packages?id=c93f83757a8fcbeb294e43b43eec0725963005be'>packages</a>/<a href='/guix/log/gnu/packages/chemistry.scm?id=c93f83757a8fcbeb294e43b43eec0725963005be'>chemistry.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/packages/chemistry.scm?id=c93f83757a8fcbeb294e43b43eec0725963005be&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-05-31 14:53:34 -0400'>2022-05-31</span></td><td><a href='/guix/commit/gnu/packages/chemistry.scm?id=5ab0cbd0f1e0fa7cca3052a51f032460fb3bb17c'>gnu: Remove python2-numpy.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/python-xyz.scm (python2-numpy): Delete variable.
(python2-numpy-1.8): Likewise.
(python-numpy)[properties]: Delete field.
* gnu/packages/maths.scm (flann): Delete comment.
* gnu/packages/chemistry.scm (with-numpy-1.8): Delete procedure.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2022-05-31 14:52:22 -0400'>2022-05-31</span></td><td><a href='/guix/commit/gnu/packages/chemistry.scm?id=99d5c9b54afb78579b6a8f2e431195e7ca8cb1e5'>gnu: Remove nmoldyn.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/chemistry.scm (nmoldyn): Delete variable.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2022-05-31 14:52:22 -0400'>2022-05-31</span></td><td><a href='/guix/commit/gnu/packages/chemistry.scm?id=dbac9f890e3c12b9d82aa00604c779f0770b8299'>gnu: Remove domainfinder.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/chemistry.scm (domainfinder): Delete variable.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2021-12-13 17:48:25 +0100'>2021-12-13</span></td><td><a href='/guix/commit/gnu/packages/chemistry.scm?id=8394619baceb118df92e355377fd543bb1aa501a'>gnu: Simplify package inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>This commit was obtained by running:

  ./pre-inst-env guix style

without any additional argument.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-10-12 17:46:23 +0000'>2021-10-12</span></td><td><a href='/guix/commit/gnu/packages/chemistry.scm?id=a1eca979fb8da842e73c42f4f53be29b169810f2'>Merge remote-tracking branch 'origin/master' into core-updates-frozen.</a></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2021-09-20 16:01:08 +0200'>2021-09-20</span></td><td><a href='/guix/commit/gnu/packages/chemistry.scm?id=cbb76da1f6fd9a27bb1826274493cc6f33000f18'>gnu: Add python-pymol.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/chemistry.scm (python-pymol): New variable.
</span></span></td><td>Ricardo Wurmus</td></tr>