doc: gh_utils: ignore added-but-not-committed files

If the file is added to the index, but not committed yet, the 'git'
command will return an empty string. This patch checks for this case and
treats it as if the file was not tracked at all.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>

doc/_extensions/zephyr/gh_utils.py

@@ -211,6 +211,8 @@ def git_info_filter(app: Sphinx, pagename) -> tuple[str, str] | None:
             .decode("utf-8")
             .strip()
         )
+        if not date_and_sha1: # added but not committed
+            return None
         date, sha1 = date_and_sha1.split(" ", 1)
         date_object = datetime.fromtimestamp(int(date))
         last_update_fmt = app.config.html_last_updated_fmt