fix(model-server-lib): don't throw exception for missing reference links

reference(...) didn't check before if the reference exists on the concept and children(...) still
doesn't.
Without these checks changes in the meta-model can result in hard to debug issues, but since the
ModelQL will be replaced soon with a type-safe alternative, restoring the old behavior is just a
temporary fix.

https://issues.modelix.org/issue/MODELIX-435

Author: slisson

model-server-lib/src/main/kotlin/org/modelix/model/server/light/QueryImplementation.kt

@@ -13,10 +13,7 @@
  */
 package org.modelix.model.server.light
 
-import org.modelix.model.api.INode
-import org.modelix.model.api.INodeReferenceSerializer
-import org.modelix.model.api.getAncestors
-import org.modelix.model.api.getDescendants
+import org.modelix.model.api.*
 import org.modelix.model.server.api.AndFilter
 import org.modelix.model.server.api.ContainsOperator
 import org.modelix.model.server.api.EndsWithOperator
@@ -53,7 +50,18 @@ fun RootOrSubquery.queryNodes(node: INode): Sequence<INode> {
         is QueryChildren -> node.getChildren(this.role).asSequence()
         is QueryDescendants -> node.getDescendants(false)
         is QueryParent -> listOfNotNull(node.parent).asSequence()
-        is QueryReference -> listOfNotNull(node.getReferenceTarget(this.role)).asSequence()
+        is QueryReference -> {
+            val link = try {
+                node.resolveReferenceLink(this.role)
+            } catch (ex: Exception) {
+                null
+            }
+            if (link == null) {
+                emptySequence()
+            } else {
+                listOfNotNull(node.getReferenceTarget(link)).asSequence()
+            }
+        }
         is QueryReferences -> node.getAllReferences()
         is QueryReferencesAndChildren -> node.getReferencesAndChildren(recursive)
         is QueryById -> {