Wrong handling of graph in SPARQL-execution

[gfobe]

In Erfurt_Store_Adapter_Virtuoso there are multiple issues when executing a sparql-query.

Erfurt_Store_Adapter_Virtuoso::_execSparql():
When $graphUri = null, in Virtuoso 06.01.3127 odbc_exec() rises an exception "Fatal error: Out of memory (allocated 10485760) (tried to allocate 18446744073709551613 bytes)" (nearly 2^64 on 64bit-system). This is, because Virtuoso comes in trouble (also in ISQL console) when a graph named 'NULL' passed to DB.DBA.SPARQL_EVAL(). So it's only possible to quote the graph name when it's not null, otherwise real NULL must be passed as this parameter to virtuoso.

Erfurt_Store_Adapter_Virtuoso::_execSparqlUpdate():
Here we also have the param $graphUri, also used nowhere in the class like in Erfurt_Store_Adapter_Virtuoso::_execSparql(). The default graph definition is created in $graphSpec, but not in use. Is it possible to add a comment why to use DB.DBA.SPARQL_EVAL in _execSparql(), but a simple SPARQL in _execSparqlUpdate()? Furthermore the PHPDoc is missing for this method.