no_useless_concat_operator should work with heredoc/nowdoc too

[mvorisek]

Feature request

Expected output:

-$a = 'test' . <<<'EOD'
-2
+$a = <<<'EOD'
+test2
 EOD;

single quoted string is basically nowdoc, double quoted heredoc

[mvorisek]

also it should remove empty string even if the syntax is different, example:

$a = "" . 'foo';
$a = 'foo';