Don't set a label if there is none

src/convert/__init__.py

@@ -138,8 +138,9 @@ class GemParser:
         # This is a pre start tag
         if starts_with_pre and self.mstate != StateEnum.PRETEXT:
             self.mstate = StateEnum.PRETEXT
-            rstring += self.get_start_tag().format(' aria-label="{}"'.format(
-                line[4:].rstrip('\n')))
+            label: str = line[4:].rstrip('\n').strip()
+            rstring += self.get_start_tag().format(
+                ' aria-label="{}"'.format(label) if label else '')
             return rstring
         # This is in pre formatted text
         elif self.mstate == StateEnum.PRETEXT and not starts_with_pre: