Fix migration table exist

Author: zonky2

src/Migration/AllowNullMigration.php

@@ -121,6 +121,10 @@ private function fetchNonNullableColumns(): array
 
         $result = [];
         foreach ($langColumns as $tableName => $tableColumnNames) {
+            if (!$schemaManager->tablesExist([$tableName])) {
+                continue;
+            }
+
             /** @var Column[] $columns */
             $columns = [];
             // The schema manager return the column list with lowercase keys, wo got to use the real names.