fix: prevent initialization failure when theme directory already exists (#6889)

#### What type of PR is this?
/kind bug
/area core
/milestone 2.20.x

#### What this PR does / why we need it:
修复从旧版本升级到 2.20 会因为默认主题目录已经存在而无法初始化的问题

#### Which issue(s) this PR fixes:
Fixes #6887

#### Does this PR introduce a user-facing change?
```release-note
修复从旧版本升级到 2.20 会因为默认主题目录已经存在而无法初始化的问题
```

application/src/main/java/run/halo/app/theme/service/ThemeServiceImpl.java

@@ -47,6 +47,7 @@ import run.halo.app.extension.ReactiveExtensionClient;
 import run.halo.app.extension.Unstructured;
 import run.halo.app.infra.SystemVersionSupplier;
 import run.halo.app.infra.ThemeRootGetter;
+import run.halo.app.infra.exception.ThemeAlreadyExistsException;
 import run.halo.app.infra.exception.ThemeUpgradeException;
 import run.halo.app.infra.exception.UnsatisfiedAttributeValueException;
 import run.halo.app.infra.properties.HaloProperties;
@@ -86,6 +87,10 @@ public class ThemeServiceImpl implements ThemeService {
                 log.warn("Failed to initialize theme from {}", location, e);
                 return Mono.empty();
             })
+            .onErrorResume(ThemeAlreadyExistsException.class, e -> {
+                log.warn("Failed to initialize theme from {}, because it already exists", location);
+                return Mono.empty();
+            })
             .then();
     }