[BingSearch] Make the bridge compatible with PHP 5.6 (#1084)
[BingSearch] Make the bridge compatible with PHP 5.6 (#1084)

* [BingSearch] Make the bridge compatible with PHP 5.6

The use of isset() with an expression is not possible in PHP 5.6. I
fixed it by replacing isset() with "null !== ".
kranack/rss-bridge,Frenzie/rss-bridge,RSS-Bridge/rss-bridge,Frenzie/rss-bridge,kranack/rss-bridge,Riduidel/rss-bridge,ORelio/rss-bridge,kranack/rss-bridge,ORelio/rss-bridge,kranack/rss-bridge,ORelio/rss-bridge,Riduidel/rss-bridge,Riduidel/rss-bridge,sebsauvage/rss-bridge,RSS-Bridge/rss-bridge,RSS-Bridge/rss-bridge