Fix for avatar image not being saved in disk cache on GTK version

Symptom
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Avatar icon in the desktop notification (libnotify) of GTK version was missing.

Cause
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Save to disk operation was aborted due to an exception in the `get_prefs`
function on agent.py, as it was trying to fetch a non-existing element
(proxy_type) from config.settings dictionary.

Solution
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Check if the key exists in the dictrionary before using it, and return an empty
string if not.