Add CheckForNull to ScmProvider#revision

sonar-plugin-api/src/main/java/org/sonar/api/batch/scm/ScmProvider.java

    * for example the SHA1 of the current HEAD in a Git branch.
    * @since 7.0
    */
+  @CheckForNull
   public String revisionId(Path path) {
     throw new UnsupportedOperationException(formatUnsupportedMessage("Getting revision id"));
   }