Helper for finding LabelValuePairs by language + label

[donaldgray]

Added TryGetValue() helper method that makes it easier to lookup List<LableValuePair> by label and language. Followed same approach as `Dictionary<T1, T2>.TryGetValue().

e.g.

if (manifest.Metadata.TryGetValue("en", "My Metadata", out var myMedataValues))
{
  // do something with myMedataValues
}

Also added null check in SanitiseHtml() method ecountered during testing.