Saxon XSLT and XQuery Processor

In backwards compatibilility mode, when a function is
declared with an optional string parameter, and the
supplied value is an empty sequence, this should be
converted to an empty string. Saxon is supplying an
empty sequence.

The result is that a test such as

string-length(@x) = 0

is returning () rather than true when @x is absent. In
most boolean contexts, () is taken as false.

Test case: predicate58

Source code fixed.