Issue title: Error installing Symphony 3.0.0 in PHP 7.3.7
 Issue body: https://github.com/symphonycms/symphonycms/blob/ea690f74cbc5b325014e45fb44b7ad5501b7da5e/install/lib/class.installerpage.php#L355

Affected Symphony version(s) : 3.0.0
PHP version(s) : 7.3.16-1+ubuntu18.04.1+deb.sury.org+1
MySQL version(s): Ver 14.14 Distrib 5.7.29, for Linux (x86_64) using  EditLine wrapper
OS(es) : Ubuntu 18.04.4 LTS (GNU/Linux 4.15.0-47-generic x86_64)

When attempting to install Symphony 3.0 on a DigitalOcean droplet running Ubuntu 18.04.4. I encountered the following error.

    A severe error occurred whilst trying to handle an exception, check the Symphony log for more details
    Undefined index: user on 355 of file 
    /var/www/example.com/public_html/install/lib/class.installerpage.php

I encountered the same error when trying to install on a localhost (same environment as in [this issue](https://github.com/symphonycms/symphonycms/issues/2910)) running macOS Catalina 10.15.3 with PHP 7.3.7 and MySQL 5.7.26 (Homebrew). I was able to successfully install Symphony 2.7.10, then [updated](https://github.com/symphonycms/symphonycms/blob/master/.docs/dev/UPDATING.md) to Symphony 3.0.0.
 Comments: 
 Comment 0: Thanks @bauhouse for all those tests !
I think you are running with all errors (which is a good thing) but I something test (sadly) without the notices.

I assume you are getting the error when loading the installer (i.e. the install process has not yet begun?)
 Comment 1: @nitriques, I am having the same error that @bauhouse is experiencing when running on Ubuntu 18.04, PHP 7.4.3, MySQL 5.7.26. This is on the installer page. Same exact error.
 Comment 2: Thanks @bzerangue  for reporting. 
 Comment 3: I also have this error with PHP 7.3.15 for the installer.  
There are 2 errors in the log:

```
2020-04-01T10:20:39+00:00 5e846a > Notice: ErrorHandler 8: Undefined index: user on line 355 of file /Users/xxx/Sites/symphonycms3/install/lib/class.installerpage.php
2020-04-01T10:20:39+00:00 5e846a > Notice: ErrorHandler 8: Use of undefined constant SYMPHONY_URL - assumed 'SYMPHONY_URL' on line 193 of file /Users/xxx/Sites/symphonycms3/symphony/lib/core/class.exceptionrenderer.php
```

 Comment 4: dump of the fields
```
array(10) { ["admin"]=> array(2) { ["max_upload_size"]=> string(7) "5242880" ["upload_blacklist"]=> string(32) "/\.(?:php[34567s]?|phtml|pht)$/i" } ["symphony"]=> array(12) { ["admin-path"]=> string(8) "symphony" ["pagination_maximum_rows"]=> string(2) "20" ["association_maximum_rows"]=> string(1) "5" ["lang"]=> string(2) "en" ["pages_table_nest_children"]=> string(2) "no" ["version"]=> string(5) "3.0.0" ["cookie_prefix"]=> string(4) "sym-" ["session_gc_divisor"]=> string(2) "10" ["session_strict_domain"]=> string(3) "yes" ["cell_truncation_length"]=> string(2) "75" ["enable_xsrf"]=> string(3) "yes" ["error_reporting_all"]=> string(2) "no" } ["log"]=> array(3) { ["archive"]=> string(1) "1" ["maxsize"]=> string(8) "10485760" ["filter"]=> int(24575) } ["database"]=> array(12) { ["host"]=> string(9) "localhost" ["port"]=> string(4) "3306" ["user"]=> NULL ["password"]=> NULL ["db"]=> NULL ["driver"]=> string(5) "mysql" ["charset"]=> string(7) "utf8mb4" ["collate"]=> string(18) "utf8mb4_unicode_ci" ["engine"]=> string(6) "InnoDB" ["tbl_prefix"]=> string(4) "sym_" ["query_caching"]=> string(2) "on" ["query_logging"]=> string(2) "on" } ["public"]=> array(1) { ["display_event_xml_in_source"]=> string(2) "no" } ["general"]=> array(2) { ["sitename"]=> string(12) "Symphony CMS" ["useragent"]=> string(14) "Symphony/3.0.0" } ["file"]=> array(1) { ["write_mode"]=> string(4) "0644" } ["directory"]=> array(1) { ["write_mode"]=> string(4) "0755" } ["region"]=> array(4) { ["time_format"]=> string(5) "g:i a" ["date_format"]=> string(5) "m/d/Y" ["datetime_separator"]=> string(1) " " ["timezone"]=> NULL } ["cache_driver"]=> array(1) { ["default"]=> string(8) "database" } }
```

 Comment 5: @wdebusschere Thanks, I've found what is causing the problem. Sadly, I lost many hours trying to install rector, in the hope that it would fix the problem by itself, but it turns out, it does not fix that. So I resorted in fixing the code with a nasty regexp.

Find: `, \$fields([^,)]+)`
Replace with: `, isset($fields$1) ? $fields$1 : null`

Ahaha. rector does a super job to remove those isset() ternary with `??`, but for now, all the code base uses this pattern.
{'source': 'GithubIssues', 'repo_name_with_owner': 'symphonycms/symphonycms', 'issue_id': 589852981, 'repo_stars': 543.0}