Fix empty links breaking backslash newlines (#648)

lib/markdown2.py

@@ -1464,6 +1464,10 @@ def _protect_url(self, url: str) -> str:
         and then hashes the now "safe" URL to prevent other safety mechanisms from tampering
         with it (eg: escaping "&" in URL parameters)
         '''
+        if not url:
+            # ignore links with empty URLs. Don't bother putting a hash there because then we'll have
+            # `"''"": "md5-..."` as an entry in the escape table, and that will cause havok
+            return url
         data_url = self._data_url_re.match(url)
         charset = None
         if data_url is not None:

test/tm-cases/backslash_escape_empty_links.html

@@ -0,0 +1,4 @@
+<p><a href="">link</a></p>
+
+<p>one<br />
+two</p>

test/tm-cases/backslash_escape_empty_links.opts

@@ -0,0 +1 @@
+{"extras": {"breaks": {"on_backslash": True}}}

test/tm-cases/backslash_escape_empty_links.text

@@ -0,0 +1,4 @@
+[link]()
+
+one\
+two