SPARQL: Do not use nested subquery for EXISTS / NOT EXISTS
SPARQL: Do not use nested subquery for EXISTS / NOT EXISTS

SQLite cannot handle a subquery with a nested correlated subquery.

Fixes NB#211928.

outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker