Fix Java 8 API compatibility

bukkit/src/main/java/com/github/games647/fastlogin/bukkit/PremiumPlaceholder.java

@@ -25,6 +25,7 @@
  */
 package com.github.games647.fastlogin.bukkit;
 
+import java.util.Collections;
 import java.util.List;
 
 import me.clip.placeholderapi.expansion.PlaceholderExpansion;
@@ -49,7 +50,7 @@ public class PremiumPlaceholder extends PlaceholderExpansion {
 
     @Override
     public @NotNull List<String> getPlaceholders() {
-        return List.of(PLACEHOLDER_VARIABLE);
+        return Collections.singletonList(PLACEHOLDER_VARIABLE);
     }
 
     @Override

bukkit/src/main/java/com/github/games647/fastlogin/bukkit/command/ToggleCommand.java

@@ -81,7 +81,7 @@ public abstract class ToggleCommand implements CommandExecutor {
             plugin.getBungeeManager().sendPluginMessage((PluginMessageRecipient) invoker, message);
         } else {
             Optional<? extends Player> optPlayer = Bukkit.getServer().getOnlinePlayers().stream().findFirst();
-            if (optPlayer.isEmpty()) {
+            if (!optPlayer.isPresent()) {
                 plugin.getLog().info("No player online to send a plugin message to the proxy");
                 return;
             }