Only batch update in case list is not empty (#747)

Apparently "BatchUpdateStatement" can't handle an empty data set

server/src/main/kotlin/suwayomi/tachidesk/manga/impl/Chapter.kt

@@ -175,6 +175,7 @@ object Chapter {
                 }
             }
 
+            if (chaptersToUpdate.isNotEmpty()) {
                 BatchUpdateStatement(ChapterTable).apply {
                     chaptersToUpdate.forEach {
                         addBatch(EntityID(it.id, ChapterTable))
@@ -188,6 +189,7 @@ object Chapter {
                     }
                     execute(this@transaction)
                 }
+            }
 
             MangaTable.update({ MangaTable.id eq mangaId }) {
                 it[MangaTable.chaptersLastFetchedAt] = Instant.now().epochSecond