pkginfo: Initialize cached strings to None

In the getter functions, cached strings are evaluated against
None, not an empty string, to determine return values.

This prevents flatpaks without appdata from having a blank name
in mintupdate's updates list (Example: org.flatpak.Builder).

usr/lib/python3/dist-packages/mintcommon/installer/pkgInfo.py

@@ -61,9 +61,9 @@ def __init__(self, pkg_hash=None):
         self.commit = ""
         self.remote_url = ""
         # We need these at minimum for a nice startup state before appstream is loaded.
-        self.cached_display_name = ""
-        self.cached_summary = ""
-        self.cached_icon = ""
+        self.cached_display_name = None
+        self.cached_summary = None
+        self.cached_icon = None
 
         # Display info fetched by methods always
         self.display_name = None