AuthorityUtils Null Check

Fixes: gh-6773

Author: JokerSun

core/src/main/java/org/springframework/security/core/authority/AuthorityUtils.java

@@ -24,6 +24,7 @@
 
 import org.springframework.security.core.GrantedAuthority;
 import org.springframework.util.StringUtils;
+import org.springframework.util.Assert;
 
 /**
  * Utility method for manipulating <tt>GrantedAuthority</tt> collections etc.
@@ -55,6 +56,7 @@ public static List<GrantedAuthority> commaSeparatedStringToAuthorityList(
 	 */
 	public static Set<String> authorityListToSet(
 			Collection<? extends GrantedAuthority> userAuthorities) {
+		Assert.notNull(userAuthorities, "userAuthorities cannot be null");
 		Set<String> set = new HashSet<>(userAuthorities.size());
 
 		for (GrantedAuthority authority : userAuthorities) {