libcupsfilters: Let filterChain() pass through data on empty chain

libcupsfilters: Let filterChain() pass through data on empty chain

If the supplied filter chain does not contain any filter or only
filter entries with the filter function set to NULL, we pass the data
through unchanged now, we simply not applying any filters.

This can happen when the universal() filter function is called iwth
the same MIME type for the input and for the output format
supplied. In such a case we simply want the data passed through.