API: Allow to pass whitespaces in MultiValue
This allows to response with an invalidreason instead silently ignore
the parameter.
Example request: api.php?format=json&action=query&titles=%20
Response before this change:
{
"batchcomplete": ""
}
Response with this change:
{
"batchcomplete": "",
"query": {
"pages": {
"-1": {
"title": " ",
"invalidreason": "The requested page title is empty or contains only the name of a namespace.",
"invalid": ""
}
}
}
}
Bug: T185846
Change-Id: I6fdaf32792a0e6e37b08176f975c10607093351b