修复作者、logo可能为空问题

src/main/kotlin/top/jie65535/jcf/MessageHandler.kt

@@ -109,7 +109,7 @@ class MessageHandler(
         val selectedMod = handlePagedList(pagedList) { mod ->
             PlainText(
                 """
-                    [${mod.name}] by ${mod.authors[0].name}
+                    [${mod.name}] by ${mod.authors.firstOrNull()?.name}
                     ${formatCount(mod.downloadCount)} Downloads  Updated ${mod.dateModified.toLocalDate()}
                     ${mod.summary}
                     ${mod.links.websiteUrl}

src/main/kotlin/top/jie65535/jcf/model/mod/Mod.kt

@@ -77,7 +77,7 @@ class Mod(
     /**
      * The mod's logo asset
      */
-    val logo: ModAsset,
+    val logo: ModAsset?,
 
     /**
      * List of screenshots assets