Complete the empty since tag (#11371)

* Complete the empty since tag

client/src/main/java/com/vaadin/client/ui/VAbstractCalendarPanel.java

     /**
      * Returns the value of initialRenderDone
      *
-     * @since
+     * @since 8.7
      */
     public boolean isInitialRenderDone() {
         return initialRenderDone;

client/src/main/java/com/vaadin/client/ui/VNativeSelect.java

     /**
      * Returns true if empty selection is allowed.
      *
-     * @since
+     * @since 8.7
      * @return empty selection is allowed
      */
     public boolean isEmptySelectionAllowed() {
     /**
      * Sets true if empty selection is allowed.
      *
-     * @since
+     * @since 8.7
      * @param emptySelectionAllowed
      */
     public void setEmptySelectionAllowed(boolean emptySelectionAllowed) {

client/src/main/java/com/vaadin/client/widget/escalator/RowContainer.java

     /**
      * For internal use only. May be removed or replaced in the future.
      *
-     * @since
+     * @since 8.7
      * @return {@code true} if row height calculations have been scheduled
      */
     public boolean isAutodetectingRowHeightLater();

compatibility-client/src/main/java/com/vaadin/v7/client/widget/escalator/RowContainer.java

      * Any extra content, such as spacers for the body, should not be included
      * in this count.
      *
-     * @since
+     * @since 8.7
      *
      * @return the actual DOM count of rows
      */

server/src/main/java/com/vaadin/event/FocusShortcut.java

  * invoked.
  *
  * @author Vaadin Ltd
- * @since
+ * @since 8.7
  */
 public class FocusShortcut extends ShortcutListener {
     protected Focusable focusable;

server/src/main/java/com/vaadin/server/ErrorHandlingRunnable.java

      * returned. If the runnable does not extend {@link ErrorHandlingRunnable},
      * then the original exception is returned.
      *
-     * @since
+     * @since 8.7
      * @param runnable
      *            the runnable for which the exception should be processed, not
      *            <code>null</code>

server/src/main/java/com/vaadin/server/KeyMapper.java

      *
      * @param objects
      *            new objects set needs to be merged.
-     * @since
+     * @since 8.7
      */
     public void merge(Set<V> objects) {
         final Set<String> keys = new HashSet<>(keyObjectMap.size());