doc: fix broken link-roles on west>=0.8

Fix broken links generated by link-roles after west was upgrade to
0.8.0. `url` was previously `None` and now returns an empty str.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>

doc/extensions/zephyr/link-roles.py

@@ -34,7 +34,7 @@ def setup(app):
         baseurl = None
 
     # or fallback to default
-    if baseurl is None:
+    if baseurl is None or baseurl == '':
         baseurl = 'https://github.com/zephyrproject-rtos/zephyr'
 
     app.add_role('zephyr_file', autolink('{}/blob/{}/%s'.format(baseurl, rev)))