fix: Re-add handbook_auth.html

its still used by handbook authenticator; i won't judge anyone if they're still loading the handbook

src/main/java/emu/grasscutter/Grasscutter.java

@@ -150,7 +150,6 @@ public final class Grasscutter {
             httpServer.addRouter(AuthenticationHandler.class);
             httpServer.addRouter(GachaHandler.class);
             httpServer.addRouter(DocumentationServerHandler.class);
-            httpServer.addRouter(HandbookHandler.class);
         }
 
         // Check if the HTTP server should start.

src/main/java/emu/grasscutter/server/http/documentation/HandbookHandler.java

@@ -14,6 +14,7 @@ import java.util.*;
 import java.util.concurrent.*;
 
 /** Handles requests for the new GM Handbook. */
+@Deprecated(since = "1.7.5-dev")
 public final class HandbookHandler implements Router {
     private String handbook;
     private final boolean serve;

src/main/java/emu/grasscutter/utils/FileUtils.java

@@ -199,10 +199,10 @@ public final class FileUtils {
     }
 
     public static byte[] readResource(String resourcePath) {
-        try (InputStream is = Grasscutter.class.getResourceAsStream(resourcePath)) {
-            return is.readAllBytes();
+        try (var is = Grasscutter.class.getResourceAsStream(resourcePath)) {
+            return Objects.requireNonNull(is).readAllBytes();
         } catch (Exception exception) {
-            Grasscutter.getLogger().warn("Failed to read resource: " + resourcePath);
+            Grasscutter.getLogger().warn("Failed to read resource: {}", resourcePath);
             Grasscutter.getLogger().debug("Failed to load resource: " + resourcePath, exception);
         }