Fix invalid username/uuid in static PlayerSkin methods

src/main/java/net/minestom/server/entity/PlayerSkin.java

@@ -25,6 +25,7 @@ public record PlayerSkin(String textures, String signature) {
     @Blocking
     public static @Nullable PlayerSkin fromUuid(@NotNull String uuid) {
         final JsonObject jsonObject = MojangUtils.fromUuid(uuid);
+        if (jsonObject == null) return null;
         final JsonArray propertiesArray = jsonObject.get("properties").getAsJsonArray();
         for (JsonElement jsonElement : propertiesArray) {
             final JsonObject propertyObject = jsonElement.getAsJsonObject();
@@ -46,6 +47,7 @@ public record PlayerSkin(String textures, String signature) {
     @Blocking
     public static @Nullable PlayerSkin fromUsername(@NotNull String username) {
         final JsonObject jsonObject = MojangUtils.fromUsername(username);
+        if (jsonObject == null) return null;
         final String uuid = jsonObject.get("id").getAsString();
         // Retrieve the skin data from the mojang uuid
         return fromUuid(uuid);

src/test/java/net/minestom/server/entity/PlayerSkinTest.java

@@ -0,0 +1,21 @@
+package net.minestom.server.entity;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+public class PlayerSkinTest {
+
+    @Test
+    public void validName() {
+        var skin = PlayerSkin.fromUsername("jeb_");
+        assertNotNull(skin);
+    }
+
+    @Test
+    public void invalidName() {
+        var skin = PlayerSkin.fromUsername("jfdsa84vvcxadubasdfcvn");
+        assertNull(skin);
+    }
+}

src/test/java/net/minestom/server/utils/TestMojangUtils.java

@@ -16,6 +16,6 @@ public class TestMojangUtils {
     @Test
     public void testInvalidNameReturnsNull() {
         var result = MojangUtils.fromUsername("jfdsa84vvcxadubasdfcvn"); // Longer than 16, always invalid
-        assert result == null;
+        assertNull(result);
     }
 }