add convenience method

gradle.properties

@@ -1,5 +1,5 @@
 group = org.xbib
 name = marc
-version = 2.9.7
+version = 2.9.8
 
 org.gradle.warning.mode = ALL

src/main/java/org/xbib/marc/MarcField.java

@@ -451,6 +451,20 @@ public class MarcField implements Comparable<MarcField> {
             return this;
         }
 
+        /**
+         * Convenience method to add a subfield only if the value is not null.
+         * @param subfieldId the dubfield ID
+         * @param value the subfield value
+         * @return this builder
+         */
+        public Builder subfieldIfNotNull(String subfieldId, String value) {
+            if (value != null) {
+                String id = validator.validateSubfieldId(subfieldId);
+                subfields.add(new Subfield(id, value));
+            }
+            return this;
+        }
+
         /**
          * Add subfield ID without a value.
          * @param subfieldId the subfield ID