fix: convert empty strings to null for integer/bool columns

MySQL strict mode rejects empty string '' for integer columns like
is_internal. Convert '' and false to null before SQL insert.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

files/custom/Espo/Modules/DataMigration/Services/EntityImporter.php

@@ -348,6 +348,11 @@ class EntityImporter
                 $value = json_encode($value, JSON_UNESCAPED_UNICODE);
             }
 
+            // Fix empty strings for integer/bool columns (MySQL strict mode)
+            if ($value === '' || $value === false) {
+                $value = null;
+            }
+
             $columns[] = "`{$column}`";
             $values[] = $value;
             $placeholders[] = '?';