Resolve variant == null warning

Author: leonardo-lemos

src/Views/FiltersView.vala

@@ -182,7 +182,7 @@ public class Display.FiltersView : Gtk.Box {
                     return new Variant ("s", "none");
                 }
 
-                return null;
+                return new Variant.maybe (VariantType.STRING, null);
             },
             null, null
         );
@@ -198,7 +198,7 @@ public class Display.FiltersView : Gtk.Box {
                     return new Variant ("s", "protanopia");
                 }
 
-                return null;
+                return new Variant.maybe (VariantType.STRING, null);
             },
             null, null
         );
@@ -214,7 +214,7 @@ public class Display.FiltersView : Gtk.Box {
                     return new Variant ("s", "protanopia-high-contrast");
                 }
 
-                return null;
+                return new Variant.maybe (VariantType.STRING, null);
             },
             null, null
         );
@@ -230,7 +230,7 @@ public class Display.FiltersView : Gtk.Box {
                     return new Variant ("s", "deuteranopia");
                 }
 
-                return null;
+                return new Variant.maybe (VariantType.STRING, null);
             },
             null, null
         );
@@ -246,7 +246,7 @@ public class Display.FiltersView : Gtk.Box {
                     return new Variant ("s", "deuteranopia-high-contrast");
                 }
 
-                return null;
+                return new Variant.maybe (VariantType.STRING, null);
             },
             null, null
         );
@@ -262,7 +262,7 @@ public class Display.FiltersView : Gtk.Box {
                     return new Variant ("s", "tritanopia");
                 }
 
-                return null;
+                return new Variant.maybe (VariantType.STRING, null);
             },
             null, null
         );
@@ -278,7 +278,7 @@ public class Display.FiltersView : Gtk.Box {
                     return new Variant ("s", "none");
                 }
 
-                return null;
+                return new Variant.maybe (VariantType.STRING, null);
             },
             null, null
         );