FINERACT-1724: Client search API fix when external ID is null

fineract-provider/src/main/java/org/apache/fineract/infrastructure/core/jersey/converter/ExternalIdJsonConverter.java

@@ -42,6 +42,8 @@ public class ExternalIdJsonConverter implements JsonConverter<ExternalId> {
     public void convertToJson(ExternalId value, JsonGenerator generator) throws IOException {
         if (value != null && !value.isEmpty()) {
             generator.writeString(value.getValue());
+        } else {
+            generator.writeNull();
         }
     }