remove unnecessary if branch related too seq construction

Not needed anymore, since seqs can never be nil nowadays.

yaml/serialization.nim

@@ -1351,8 +1351,6 @@ proc load*[K](input: Stream | string, target: var K)
     else: internalError("Unexpected exception: " & $e.parent.name)
 
 proc loadMultiDoc*[K](input: Stream | string, target: var seq[K]) =
-  if target.len == 0:
-    target = newSeq[K]()
   var
     parser = newYamlParser(serializationTagLibrary)
     events = parser.parse(input)