fix: 🐛 Make Commit Author envs optional

In some pipelines the commit author is not available (i.e. cron). Use "Woodpecker" and Woodpecker's Favicon as a placeholder if the author is missing

src/discord_notifier/__main__.py

@@ -77,8 +77,8 @@ webhook.add_embed(DiscordEmbed(
         "url": environ["CI_REPO_URL"]
     },
     footer={
-        "text": environ["CI_COMMIT_AUTHOR"],
-        "icon_url": environ["CI_COMMIT_AUTHOR_AVATAR"]
+        "text": environ.get("CI_COMMIT_AUTHOR", "Woodpecker"),
+        "icon_url": environ.get("CI_COMMIT_AUTHOR_AVATAR", "https://ci.shielddagger.com/favicons/favicon-dark-default.png")
     },
     thumbnail={
         "url": getenv("PLUGIN_ICON_URL"),