Fix flatpak list_apps when no apps are installed

src/decman/plugins/flatpak.py

@@ -176,6 +176,7 @@ class FlatpakCommands:
 
         If ``as_user`` is ``True``, run the command as the user for whom packages are removed.
         """
+
         return [
             "flatpak",
             "remove",
@@ -220,6 +221,10 @@ class FlatpakInterface:
         )
         packages = packages_text.strip().split("\n")
 
+        # In case no apps are installed, the list contains this
+        if "" in packages:
+            packages.remove("")
+
         return set(packages)
 
     def install(self, packages: set[str], user: str | None = None):