make S3 tests to run when S3 is statically linked

* use the environment variable HA_S3_SO, not a literal ha_s3 in cnf files
* make ConfigFactory to support empty option values
* update no_s3.result after MDEV-11412

mysql-test/lib/My/ConfigFactory.pm

@@ -347,7 +347,7 @@ sub resolve_at_variable {
     or croak "There is no group named '$group_name' that ",
       "can be used to resolve '$option_name' for test '$self->{testname}'";
 
-    my $value= $from_group->value($option_name);
+    my $value= $from_group->value($option_name) || '';
     $res .= $before.$value;
   }
   $res .= $after;

mysql-test/suite/s3/my.cnf

@@ -3,7 +3,7 @@
 
 [mysqld.1]
 plugin-maturity = alpha
-plugin-load-add=ha_s3
+plugin-load-add=@ENV.HA_S3_SO
 s3=ON
 #s3-host-name=s3.amazonaws.com
 #s3-protocol-version=Amazon

mysql-test/suite/s3/no_s3.result

@@ -9,5 +9,3 @@ ERROR 42000: Table 's3_unique_table' uses an extension that doesn't exist in thi
 rename table s3_unique_table to t1;
 ERROR HY000: Error on rename of './test/s3_unique_table' to './test/t1' (errno: 138 "Unsupported extension used for table")
 drop table s3_unique_table;
-Warnings:
-Warning	1112	Table 's3_unique_table' uses an extension that doesn't exist in this MariaDB version

mysql-test/suite/s3/slave.cnf

 [mysqld.2]
 plugin-maturity = alpha
-plugin-load-add=ha_s3
+plugin-load-add=@ENV.HA_S3_SO
 s3=ON
 #s3-host-name=s3.amazonaws.com
 #s3-protocol-version=Amazon