build: set vaadin dependency as optional

pom.xml

@@ -73,6 +73,7 @@
         <dependency>
             <groupId>com.vaadin</groupId>
             <artifactId>vaadin-core</artifactId>
+            <optional>true</optional>
             <exclusions>
                 <!-- Webjars are only needed when running in Vaadin 13 compatibility mode in V14.
                      Your add-on can support npm in the same version mode and V13 compatiblity mode at the same time,