Fix nullpointer when command requires no permissions

- Add test to verify connections among the command initialization

src/main/java/fr/xephi/authme/command/FoundCommandResult.java

@@ -108,12 +108,15 @@ public class FoundCommandResult {
      * @return True if the command sender has permission, false otherwise.
      */
     public boolean hasPermission(CommandSender sender) {
-        // Make sure the command description is valid
-        if (this.commandDescription == null)
+        if (commandDescription == null) {
             return false;
+        } else if (commandDescription.getCommandPermissions() == null) {
+            return true;
+        }
 
-        // Get and return the permission
-        return this.commandDescription.getCommandPermissions().hasPermission(sender);
+        // TODO: Move permissions check to the permission package; command package should not define permission-checking
+        // API
+        return commandDescription.getCommandPermissions().hasPermission(sender);
     }
 
     /**

src/test/java/fr/xephi/authme/command/CommandManagerTest.java

@@ -65,6 +65,28 @@ public class CommandManagerTest {
         walkThroughCommands(manager.getCommandDescriptions(), descriptionTester);
     }
 
+    /** Ensure that all children of a command stored the parent. */
+    @Test
+    public void shouldHaveConnectionBetweenParentAndChild() {
+        // given
+        BiConsumer connectionTester = new BiConsumer() {
+            @Override
+            public void accept(CommandDescription command, int depth) {
+                if (command.hasChildren()) {
+                    for (CommandDescription child : command.getChildren()) {
+                        assertThat(command.equals(child.getParent()), equalTo(true));
+                    }
+                }
+                // Checking that the parent has the current command as child is redundant as this is how we can traverse
+                // the "command tree" in the first place - if we're here, it's that the parent definitely has the
+                // command as child.
+            }
+        };
+
+        // when/then
+        walkThroughCommands(manager.getCommandDescriptions(), connectionTester);
+    }
+
     @Test
     public void shouldNotDefineSameLabelTwice() {
         // given