Update gradle.properties

fixes https://github.com/halo-dev/plugin-s3/issues/93
```release-note
修复从旧版本升级后上传文件的NPE错误
```
/kind bug

gradle.properties

@@ -1 +1 @@
-version=1.5.0-SNAPSHOT
+version=1.6.1-SNAPSHOT

src/main/java/run/halo/s3os/UrlUtils.java

@@ -21,7 +21,7 @@ public class UrlUtils {
 
     public static String findUrlSuffix(List<S3OsProperties.urlSuffixItem> urlSuffixList,
                                        String fileName) {
-        if (StringUtils.isBlank(fileName)) {
+        if (StringUtils.isBlank(fileName) || urlSuffixList == null) {
             return null;
         }
         fileName = fileName.toLowerCase();

src/test/java/run/halo/s3os/UrlUtilsTest.java

@@ -46,5 +46,10 @@ class UrlUtilsTest {
         String fileName5 = "";
         String result5 = UrlUtils.findUrlSuffix(urlSuffixList, fileName5);
         assertNull(result5);
+
+        // 测试urlSuffixList为null，期望返回null
+        String fileName6 = "example";
+        String result6 = UrlUtils.findUrlSuffix(null, fileName6);
+        assertNull(result6);
     }
 }