perf: associate ghost user with posts on user deletion. (#3293)

#### What type of PR is this?

/kind bug

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

文章发布用户被删除后，设置这篇文章的发布人为Ghost

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

Fixes #3265

```release-note
NONE
```

src/main/java/run/halo/app/content/Contributor.java

@@ -13,4 +13,11 @@ public class Contributor {
     private String displayName;
     private String avatar;
     private String name;
+
+    public static Contributor getGhost() {
+        Contributor contributor = new Contributor();
+        contributor.setName("ghost");
+        contributor.setDisplayName("已删除用户");
+        return contributor;
+    }
 }

src/main/java/run/halo/app/content/impl/PostServiceImpl.java

@@ -231,7 +231,8 @@ private Flux<Contributor> listContributors(List<String> usernames) {
                 contributor.setDisplayName(user.getSpec().getDisplayName());
                 contributor.setAvatar(user.getSpec().getAvatar());
                 return contributor;
-            });
+            })
+            .defaultIfEmpty(Contributor.getGhost());
     }
 
     @Override