GH-5121: support empty left bind join (OPTIONAL) in FedX

[aschwarte10]

GitHub issue resolved: #5121

Previously we introduced support for left bind joins in FedX. The case
of empty left bind joins (i.e. where the clause inside the OPTIONAL does
not provide any statements) was not handled and resulted in an exception

This change now adds support for empty optional joins and passes the
results from the left-handside through.

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Note: this is a follow up issue and fix for #5121, which we found by using RDF4J 5.1-M1. The fix should be added to 5.1.0

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