Fix exception on invalid username or UUID (#532)

src/main/java/net/minestom/server/utils/mojang/MojangUtils.java

@@ -39,6 +39,11 @@ public final class MojangUtils {
             try {
                 // Retrieve from the rate-limited Mojang API
                 final String response = URLUtils.getText(url);
+                // If our response is "", that means the url did not get a proper object from the url
+                // So the username or UUID was invalid, and therefore we return null
+                if(response.isEmpty()) {
+                    return null;
+                }
                 return JsonParser.parseString(response).getAsJsonObject();
             } catch (IOException e) {
                 MinecraftServer.getExceptionManager().handleException(e);

src/test/java/misc/TestMojangUtils.java

@@ -0,0 +1,21 @@
+package misc;
+
+import net.minestom.server.utils.mojang.MojangUtils;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class TestMojangUtils {
+    @Test
+    public void testValidNameWorks() {
+        var result = MojangUtils.fromUsername("jeb_");
+        assertNotNull(result);
+        assertEquals("jeb_", result.get("name").getAsString());
+    }
+
+    @Test
+    public void testInvalidNameReturnsNull() {
+        var result = MojangUtils.fromUsername("jfdsa84vvcxadubasdfcvn"); // Longer than 16, always invalid
+        assert result == null;
+    }
+}