Generic/UnnecessaryStringConcat: remove unnecessary condition

Both in PHP and JS there is not a scenario where there is no non-empty
token before T_STRING_CONCAT or T_PLUS. There will always be at least
the T_OPEN_TAG token. So checking if `$prev` is `false` is not necessary.
`$prev` will never be `false`.

Author: rodrigoprimo

src/Standards/Generic/Sniffs/Strings/UnnecessaryStringConcatSniff.php

@@ -84,7 +84,7 @@ public function process(File $phpcsFile, $stackPtr)
 
         $prev = $phpcsFile->findPrevious(T_WHITESPACE, ($stackPtr - 1), null, true);
         $next = $phpcsFile->findNext(T_WHITESPACE, ($stackPtr + 1), null, true);
-        if ($prev === false || $next === false) {
+        if ($next === false) {
             return;
         }