build: set vaadin dependency as optional

Author: javier-godoy

pom.xml

@@ -70,6 +70,7 @@
         <dependency>
             <groupId>com.vaadin</groupId>
             <artifactId>vaadin-core</artifactId>
+            <optional>true</optional>
             <exclusions>
                 <!-- Webjars are only needed when running in Vaadin 13 compatibility mode -->
                 <exclusion>