Fix javadocs for getId() and setId() (#4960)

For some reason, the javadoc was based on representing absence of id as
`null`, whereas the implementation uses empty string in the setter and
empty optional in the getter.

flow-server/src/main/java/com/vaadin/flow/component/Component.java

@@ -383,8 +383,8 @@ public Optional<UI> getUI() {
      * page.
      *
      * @param id
-     *            the id to set, or <code>null</code> to remove any previously
-     *            set id
+     *            the id to set, or <code>""</code> to remove any previously set
+     *            id
      */
     public void setId(String id) {
         set(idDescriptor, id);
@@ -395,7 +395,7 @@ public void setId(String id) {
      *
      * @see #setId(String)
      *
-     * @return the id, or <code>""</code> if no id has been set
+     * @return the id, or and empty optional if no id has been set
      */
     public Optional<String> getId() {
         return get(idDescriptor);