Do not match text markup end with empty contents.

src/parser/org_source.rs

@@ -59,6 +59,10 @@ impl<'s> OrgSource<'s> {
         self.end - self.start
     }
 
+    pub(crate) fn get_byte_offset(&self) -> usize {
+        self.start
+    }
+
     pub(crate) fn get_preceding_character(&self) -> Option<char> {
         self.preceding_character
     }