Clean invalid ext_emconf.php constraints

[ohader]

Using composer require typo3-ter/svconnector 3.0.0 leads to some strange behavior when the ext_emconf.php is being written. To make it short - not the composer installer is causing this, but the EM_CONF section from the T3X file is corrupt already.

The written ext_emconf.php in the target directory contained something like the following (see the empty constraint), which caused an exception on installing the extension in TYPO3.

    'constraints' => 
    array (
        'depends' => 
        array (
            'typo3' => '7.6.0-7.99.99',
            '' => '',
        ),