fs: return string on empty fs.promises.readFile() when encoding provided

fs: return string on empty fs.promises.readfile when encoding provided

fs.promises.readfile() would provide a buffer whenever an empty file
is read, when it should have returned an empty string when encoding
is provided.

lib/internal/fs/promises.js

@@ -139,7 +139,7 @@ async function readFileHandle(filehandle, options) {
   }
 
   if (size === 0)
-    return Buffer.alloc(0);
+    return options.encoding ? '' : Buffer.alloc(0);
 
   if (size > kMaxLength)
     throw new ERR_FS_FILE_TOO_LARGE(size);