fix: no_useless_concat_operator - do not break variable (2)

[tamiroh]

This pull request fixes a bug that was not considered in #7827.

Using Preg::match, \s also matches a full-width space. That is,

"abc $d" . "　e" // contains full-width space before 'e'

is fixed to "abc $d　e" as it corresponds to the "Allowd Pattern", but the variable $d would be broken because $d　e is treated as a single variable.

To prevent the above, use instead of \s.

[coveralls]

coverage: 96.013%. remained the same
when pulling 8aca6dc on tamiroh:fix-no-useless-concat-operator-fixer-2
into 87c75fa on PHP-CS-Fixer:master.