title: Stop enumerateAttribute:inRange:options:usingBlock: from calling my block with nil values
body: <p>I'm calling the following selector on an existing <code>NSAttributedString</code> with no <code>kCTFontAttributeName</code> ranges:</p>
  
  <pre><code>[attributedString enumerateAttribute:(NSString *) kCTFontAttributeName
                               inRange:NSMakeRange(0, [attributedString length])
                               options:NSAttributedStringEnumerationLongestEffectiveRangeNotRequired
                            usingBlock:^(id value, NSRange range, BOOL *stop) {
      NSLog(@"Attribute: %@, %@", value, NSStringFromRange(range));
  }];
  </code></pre>
  
  <p>and I get the output below, but I would expect to get no output. Suggestions?</p>
  
  <pre><code>Attribute: (null), {0, 27}
  Attribute: (null), {27, 1}
  Attribute: (null), {28, 1}
  Attribute: (null), {29, 1}
  Attribute: (null), {30, 1}
  </code></pre>