Skip null FKs

(cherry picked from commit 2285e7b)

Author: mike-podolskiy90

src/main/java/io/frictionlessdata/datapackage/resource/AbstractResource.java

@@ -411,6 +411,9 @@ public void checkRelations(Package pkg) {
                                 Map<String, String> fieldMapping = fk.getForeignKey().getFieldMapping();
                                 String refFieldName = fieldMapping.get(key);
                                 Object fkVal = row.get(key);
+                                if (fkVal == null) {
+                                    continue;
+                                }
                                 boolean found = false;
 
                                 for (Object refRow : refData) {