fix: None Type tool installation during startup

Co-Authored-By: Adam Outler <adamoutler@gmail.com>

backend/open_webui/utils/plugin.py

@@ -260,7 +260,7 @@ def install_tool_and_function_dependencies():
                 all_dependencies += f"{dependencies}, "
         for tool in tool_list:
             # Only install requirements for admin tools
-            if tool.user.role == "admin":
+            if tool.user and tool.user.role == "admin":
                 frontmatter = extract_frontmatter(replace_imports(tool.content))
                 if dependencies := frontmatter.get("requirements"):
                     all_dependencies += f"{dependencies}, "