fix: Handle None value for feeds config to prevent TypeError

Fix crash when plugin_config['feeds'] exists but is None, causing
TypeError when checking 'custom_feeds' in feeds_config.

Problem:
- When plugin_config['feeds'] exists but is None, dict.get('feeds', {})
  returns None (not the default {}) because dict.get() only uses default
  when key doesn't exist, not when value is None
- Line 3642's 'custom_feeds' in feeds_config raises TypeError because
  None is not iterable
- This can crash the API endpoint if a plugin config has feeds: null

Solution:
- Change plugin_config.get('feeds', {}) to plugin_config.get('feeds') or {}
  to ensure feeds_config is always a dict (never None)
- Add feeds_config check before 'in' operator for extra safety

This ensures the code gracefully handles feeds: null in plugin configuration.

web_interface/blueprints/api_v3.py

@@ -3638,8 +3638,8 @@ def save_plugin_config():
                 
                 # Force fix for feeds.custom_feeds if it's still a dict (fallback)
                 if 'feeds' in plugin_config:
-                    feeds_config = plugin_config.get('feeds', {})
-                    if 'custom_feeds' in feeds_config and isinstance(feeds_config['custom_feeds'], dict):
+                    feeds_config = plugin_config.get('feeds') or {}
+                    if feeds_config and 'custom_feeds' in feeds_config and isinstance(feeds_config['custom_feeds'], dict):
                         custom_feeds_dict = feeds_config['custom_feeds']
                         # Check if all keys are numeric
                         keys = list(custom_feeds_dict.keys())