fix: Fix unrecognized functional index defined in MySQL

+ The issue leads to crashing dbal.
+ The issue is fixed by skipping adding the columns, returned as null, to the index at the time of the creation of the index

Author: junaid farooq

src/Schema/Index.php

@@ -12,6 +12,7 @@
 use function array_search;
 use function array_shift;
 use function count;
+use function is_string;
 use function strtolower;
 
 class Index extends AbstractAsset
@@ -57,6 +58,10 @@ public function __construct(
         $this->_isPrimary = $isPrimary;
 
         foreach ($columns as $column) {
+            if (! is_string($column)) {
+                continue;
+            }
+
             $this->_addColumn($column);
         }