Fix DrawableFlag returns empty texture if there's no flag avaliable for needed country

osu.Game.Tests/Visual/Online/TestSceneRankingsHeader.cs

@@ -61,10 +61,17 @@ namespace osu.Game.Tests.Visual.Online
                 FullName = "Belarus"
             };
 
+            var unknownCountry = new Country
+            {
+                FlagName = "CK",
+                FullName = "Cook Islands"
+            };
+
             AddStep("Set country", () => countryBindable.Value = country);
             AddAssert("Check scope is Performance", () => scope.Value == RankingsScope.Performance);
             AddStep("Set scope to Score", () => scope.Value = RankingsScope.Score);
             AddAssert("Check country is Null", () => countryBindable.Value == null);
+            AddStep("Set country with no flag", () => countryBindable.Value = unknownCountry);
         }
     }
 }

osu.Game/Users/Drawables/DrawableFlag.cs

@@ -26,7 +26,7 @@ namespace osu.Game.Users.Drawables
             if (ts == null)
                 throw new ArgumentNullException(nameof(ts));
 
-            Texture = ts.Get($@"Flags/{country?.FlagName ?? @"__"}");
+            Texture = ts.Get($@"Flags/{country?.FlagName ?? @"__"}") ?? ts.Get($@"Flags/__");
         }
     }
 }