fixed bug, no separator when flatting array with first value by index "0"

Author: Rom-axa

src/Flatten.php

@@ -155,7 +155,7 @@ private function flattenGenerator($var, $prefix)
             }
         }
 
-        $prefix .= (empty($prefix) ? '' : $this->separator);
+        $prefix .= ($prefix === '' ? '' : $this->separator);
         foreach ($var as $key => $value) {
             foreach ($this->flattenGenerator($value, $prefix . $key) as $k => $v) {
                 yield $k => $v;

test/FlattenTest.php

@@ -58,6 +58,7 @@ public function arraysProvider()
     {
         return [
             [ [ ], [ ] ],
+            [ [ [ 'a' => 1 ], [ 'b' => 2] ], ['0.a' => 1, '1.b' => 2] ],
             [ [ 0 ], [ '0' => 0 ] ],
             [ [ 1, 2 ], [ '0' => 1, '1' => 2 ] ],
             [