fix #15148 (#15149)

* fix #15148

Co-authored-by: alaviss <leorize+oss@disroot.org>

tests/async/t15148.nim

@@ -0,0 +1,12 @@
+import asyncdispatch, asyncfile, os
+
+const Filename = "t15148.txt"
+
+proc saveEmpty() {.async.} =
+  let
+    text = ""
+    file = openAsync(Filename, fmWrite)
+  await file.write(text)
+  file.close()
+
+waitFor saveEmpty()