fix(LocalBook.saveBookFile): 保存前检查是inputstream否为空

app/src/main/java/io/legado/app/exception/EmptyFileException.kt

@@ -0,0 +1,6 @@
+package io.legado.app.exception
+
+/**
+ * 文件为空
+ */
+class EmptyFileException(msg: String) : NoStackTraceException(msg)