GH-4920 When sending the remote size query make sure we don't send null,

or RDF4J nill

core/repository/sparql/src/main/java/org/eclipse/rdf4j/repository/sparql/SPARQLConnection.java

@@ -42,6 +42,8 @@
 import org.eclipse.rdf4j.model.impl.DynamicModelFactory;
 import org.eclipse.rdf4j.model.impl.SimpleValueFactory;
 import org.eclipse.rdf4j.model.util.Literals;
+import org.eclipse.rdf4j.model.vocabulary.RDF4J;
+import org.eclipse.rdf4j.model.vocabulary.SESAME;
 import org.eclipse.rdf4j.query.BindingSet;
 import org.eclipse.rdf4j.query.BooleanQuery;
 import org.eclipse.rdf4j.query.GraphQuery;
@@ -313,12 +315,13 @@ public long size(Resource... contexts) throws RepositoryException {
 	String sizeAsTupleQuery(Resource... contexts) {
 		String query = COUNT_EVERYTHING;
 		if (contexts != null && isQuadMode()) {
-			if (contexts.length == 1 && contexts[0].isIRI()) {
-				query = "SELECT (COUNT(*) AS ?count) WHERE { GRAPH <" + ((IRI) contexts[0]).stringValue()
+			if (contexts.length == 1 && isExposableGraphIri(contexts[0])) { // in case the context is null we want the
+																			// default graph.
+				query = "SELECT (COUNT(*) AS ?count) WHERE { GRAPH <" + contexts[0].stringValue()
 						+ "> { ?s ?p ?o}}";
 			} else if (contexts.length > 0) {
 				String graphs = Arrays.stream(contexts)
-						.filter(Resource::isIRI)
+						.filter(SPARQLConnection::isExposableGraphIri)
 						.map(Resource::stringValue)
 						.map(s -> "FROM <" + s + ">")
 						.collect(Collectors.joining(" "));
@@ -328,6 +331,17 @@ String sizeAsTupleQuery(Resource... contexts) {
 		return query;
 	}
 
+	/**
+	 * For the sparql protocol a context must be an IRI However we can't send out the RDF4j intenral default graph IRIs
+	 *
+	 * @param resource to test if it can be the IRI for a named graph
+	 * @return true if it the input can be a foreign named graph.
+	 */
+	private static boolean isExposableGraphIri(Resource resource) {
+		// We use the instanceof test to avoid any issue with a null pointer.
+		return resource instanceof IRI && RDF4J.NIL != resource && SESAME.NIL != resource;
+	}
+
 	@Override
 	public RepositoryResult<Statement> getStatements(Resource subj, IRI pred, Value obj, boolean includeInferred,
 			Resource... contexts) throws RepositoryException {

core/repository/sparql/src/test/java/org/eclipse/rdf4j/repository/sparql/SPARQLConnectionTest.java

@@ -13,6 +13,7 @@
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.eclipse.rdf4j.model.util.Values.iri;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyBoolean;
@@ -32,6 +33,7 @@
 import org.eclipse.rdf4j.model.impl.SimpleValueFactory;
 import org.eclipse.rdf4j.model.vocabulary.FOAF;
 import org.eclipse.rdf4j.model.vocabulary.RDF;
+import org.eclipse.rdf4j.model.vocabulary.RDF4J;
 import org.eclipse.rdf4j.model.vocabulary.RDFS;
 import org.eclipse.rdf4j.query.impl.MapBindingSet;
 import org.eclipse.rdf4j.query.impl.SimpleBinding;
@@ -132,6 +134,16 @@ public void testSizeQuery() throws Exception {
 		sizeAsTupleQuery = subject.sizeAsTupleQuery(vf.createIRI("urn:g1"), vf.createBNode());
 		query = new SPARQLParserFactory().getParser().parseQuery(sizeAsTupleQuery, "http://example.org/");
 		assertNotNull(query);
+
+		sizeAsTupleQuery = subject.sizeAsTupleQuery(RDF4J.NIL);
+		query = new SPARQLParserFactory().getParser().parseQuery(sizeAsTupleQuery, "http://example.org/");
+		assertNotNull(query);
+		assertFalse(sizeAsTupleQuery.contains("nil"));
+
+		sizeAsTupleQuery = subject.sizeAsTupleQuery(null);
+		query = new SPARQLParserFactory().getParser().parseQuery(sizeAsTupleQuery, "http://example.org/");
+
+		assertNotNull(query);
 	}
 
 	@Test