MDEV-21088 Table cannot be loaded after instant ADD/DROP COLUMN

In MariaDB Server 10.4, btr_cur_instant_init_low() assumes that
all PRIMARY KEY columns that are internally variable-length will
be encoded in 0 bytes in the metadata record. Sometimes, CHAR
columns can be encoded as variable-length. We should not
unnecessarily reserve space for a dummy string value in the
metadata record.

storage/innobase/handler/handler0alter.cc

@@ -4278,10 +4278,17 @@ innobase_add_instant_try(
 			case MYSQL_TYPE_MEDIUM_BLOB:
 			case MYSQL_TYPE_BLOB:
 			case MYSQL_TYPE_LONG_BLOB:
+			variable_length:
 				/* Store the empty string for 'core'
 				variable-length NOT NULL columns. */
 				dfield_set_data(d, field_ref_zero, 0);
 				break;
+			case MYSQL_TYPE_STRING:
+				if (col->mbminlen != col->mbmaxlen
+				    || !dict_table_is_comp(user_table)) {
+					goto variable_length;
+				}
+				/* fall through */
 			default:
 				/* For fixed-length NOT NULL 'core' columns,
 				get a dummy default value from SQL. Note that