Skip relationshipType if target is null

Otherwise we end up with relationshipTypes none of the relationships actually has

Author: FlorentinD

cypher-aggregation/src/main/java/org/neo4j/gds/projection/CypherAggregation.java

@@ -195,11 +195,10 @@ public void update(
                 }
             }
 
-            var relImporter = this.relImporters.computeIfAbsent(relationshipType, this::newRelImporter);
-
             var intermediateSourceId = loadNode(sourceNode, sourceNodeLabels, sourceNodePropertyValues);
 
             if (targetNode != null) {
+                var relImporter = this.relImporters.computeIfAbsent(relationshipType, this::newRelImporter);
                 var intermediateTargetId = loadNode(targetNode, targetNodeLabels, targetNodePropertyValues);
 
                 if (this.relationshipPropertySchemas != null && !this.relationshipPropertySchemas.isEmpty()) {

cypher-aggregation/src/test/java/org/neo4j/gds/projection/CypherAggregationTest.java

@@ -441,6 +441,20 @@ void testRelationshipType(String type) {
             .returns(4L, Graph::relationshipCount);
     }
 
+
+    @Test
+    void ignoreRelationshipTypeIfTargetIsNull() {
+        runQuery(
+            "UNWIND [[1, null, null], [1, 2, 'REL']] AS data" +
+            " RETURN gds.alpha.graph.project('g', data[0], data[1], {}, {relationshipType: data[2]})"
+        );
+
+        var graphStore = GraphStoreCatalog.get("", db.databaseName(), "g").graphStore();
+
+        assertThat(graphStore.relationshipCount()).isEqualTo(1);
+        assertThat(graphStore.relationshipTypes()).containsOnly(org.neo4j.gds.RelationshipType.of("REL"));
+    }
+
     @Test
     void shouldNotFailOnMissingProperty() {
         var query = "MATCH (s:B)-[:REL]->(t:B) " +