Fix the problem of resolving authentication in template when using curl or ab command (#3909)

#### What type of PR is this?

/kind bug
/area core
/milestone 2.6.x

#### What this PR does / why we need it:

When we use curl or ab command to access any pages rendered by thymeleaf at first up, we will get the error described in <https://github.com/halo-dev/halo/issues/3861>.

This PR will resolve the problem by checking null value.

#### Which issue(s) this PR fixes:

Fixes https://github.com/halo-dev/halo/issues/3861

#### Does this PR introduce a user-facing change?

```release-note
解决访问页面无法渲染的问题
```

application/src/main/java/run/halo/app/theme/ReactivePropertyAccessor.java

@@ -34,7 +34,8 @@ public class ReactivePropertyAccessor implements PropertyAccessor {
         if (isReactiveType(target)) {
             return true;
         }
-        List<PropertyAccessor> propertyAccessors = context.getPropertyAccessors();
+        var propertyAccessors =
+            getPropertyAccessorsToTry(target.getClass(), context.getPropertyAccessors());
         for (PropertyAccessor propertyAccessor : propertyAccessors) {
             if (propertyAccessor.canRead(context, target, name)) {
                 return true;
@@ -83,7 +84,7 @@ public class ReactivePropertyAccessor implements PropertyAccessor {
 
     private boolean isReactiveType(Object target) {
         if (target == null) {
-            return false;
+            return true;
         }
         Class<?> clazz = target.getClass();
         return Mono.class.isAssignableFrom(clazz)