Avoid leaking system path from pg_available_extensions

Hi,

I just tested “Add paths of extensions to pg_available_extensions”, and found an issue.

This is a simple repro:
```
evantest=# reset extension_control_path;
RESET
evantest=# select * from pg_available_extensions where name = 'plpgsql';
  name   | default_version | installed_version | location |           comment
---------+-----------------+-------------------+----------+------------------------------
 plpgsql | 1.0             | 1.0               | $system  | PL/pgSQL procedural language
(1 row)

evantest=# set extension_control_path='';
SET
evantest=# select * from pg_available_extensions where name = 'plpgsql';
  name   | default_version | installed_version |             location             |           comment
---------+-----------------+-------------------+----------------------------------+------------------------------
 plpgsql | 1.0             | 1.0               | /usr/local/pgsql/share/extension | PL/pgSQL procedural language
(1 row)
```

When extension_control_path is not set, location shows “$system", which is consistent with what the documentation says:
```
       <para>
        The default value for this parameter is
        <literal>'$system'</literal>. If the value is set to an empty
        string, the default <literal>'$system'</literal> is also assumed.
       </para>
```

However, as shown above, when I set extension_control_path to an empty string, the absolute system path is displayed. I consider this an information leakage bug.

The fix is straightforward; see the attached patch for details. After the fix, when extension_control_path is an empty string, location shows “$system” now:
```
evantest=# set extension_control_path='';
SET
evantest=# select * from pg_available_extensions where name = 'plpgsql';
  name   | default_version | installed_version | location |           comment
---------+-----------------+-------------------+----------+------------------------------
 plpgsql | 1.0             | 1.0               | $system  | PL/pgSQL procedural language
(1 row)
```

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/