Return a missing value that is understandable by SortField.setMissingValue(Object).
Most implementations return null because they already replace the value at the fielddata level.
However this can't work in case of strings since there is no such thing as a string which
compares greater than any other string, so in that case we need to return
SortField.STRING_FIRST or SortField.STRING_LAST so that the coordinating node
knows how to deal with null values.