fix rememberme for OIDC

src/main/java/de/bstly/board/businesslogic/UserManager.java

@@ -96,7 +96,13 @@ public class UserManager implements UserDetailsService, SmartInitializingSinglet
 			}
 		}
 
-		return new User(username, localUser.getPasswordHash(), authorities);
+		String passwordHash = localUser.getPasswordHash();
+
+		if (passwordHash == null) {
+			passwordHash = "";
+		}
+
+		return new User(username, passwordHash, authorities);
 	}
 
 	/*