Add a defensive coding in posts per page sniff

This will prevent the undefined offset internal error.

WordPress/Sniffs/WP/PostsPerPageSniff.php

@@ -78,7 +78,7 @@ public function callback( $key, $val, $line, $group ) {
 			$val = $stripped_val;
 		}
 
-		$first_char = $val[0];
+		$first_char = isset( $val[0] ) ? $val[0] : '';
 		if ( '-' === $first_char || '+' === $first_char ) {
 			$val = ltrim( $val, '-+' );
 		} else {