Avoid empty alternative command buckets

Essentials/src/main/java/com/earth2me/essentials/AlternativeCommandsHandler.java

@@ -129,6 +129,10 @@ public Command getAlternative(final String label) {
             }
         }
 
+        if (commands.isEmpty()) {
+            return null;
+        }
+
         // return the first alias
         return commands.get(0).get();
     }

Essentials/src/test/java/com/earth2me/essentials/AlternativeCommandsHandlerTest.java

@@ -0,0 +1,47 @@
+package com.earth2me.essentials;
+
+import org.bukkit.Server;
+import org.bukkit.command.Command;
+import org.bukkit.plugin.Plugin;
+import org.bukkit.plugin.PluginManager;
+import org.junit.jupiter.api.Test;
+
+import java.lang.ref.WeakReference;
+import java.lang.reflect.Field;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+class AlternativeCommandsHandlerTest {
+
+    @Test
+    void returnsNullWhenAllAlternativeCommandsHaveBeenCollected() throws Exception {
+        final IEssentials essentials = mock(IEssentials.class);
+        final Server server = mock(Server.class);
+        final PluginManager pluginManager = mock(PluginManager.class);
+        final ISettings settings = mock(ISettings.class);
+        when(essentials.getServer()).thenReturn(server);
+        when(server.getPluginManager()).thenReturn(pluginManager);
+        when(pluginManager.getPlugins()).thenReturn(new Plugin[0]);
+        when(essentials.getSettings()).thenReturn(settings);
+        when(settings.isDebug()).thenReturn(false);
+
+        final AlternativeCommandsHandler handler = new AlternativeCommandsHandler(essentials);
+        final Field field = AlternativeCommandsHandler.class.getDeclaredField("altCommands");
+        field.setAccessible(true);
+        @SuppressWarnings("unchecked")
+        final Map<String, List<WeakReference<Command>>> alternatives =
+                (Map<String, List<WeakReference<Command>>>) field.get(handler);
+        alternatives.put("foo", new ArrayList<>(Arrays.asList(
+                new WeakReference<Command>(null),
+                new WeakReference<Command>(null)
+        )));
+
+        assertNull(handler.getAlternative("foo"));
+    }
+}