Post `check()` allows empty strings

[kylefox]

In Creating Posts the following code is added to lib/collections/posts.js to validate postAttributes:

check(postAttributes, {
  title: String,
  url: String
});

However, this provides no enforcement on string length and thus allows the user to submit an empty form, creating a document with empty attributes.

According to the Match Patterns documentation, the correct approach is to use Match.Where(condition) to ensure the string is not empty:

NonEmptyString = Match.Where(function (x) {
  check(x, String);
  return x.length > 0;
});

check(postAttributes, {
  title: NonEmptyString,
  url: NonEmptyString
});