Fix PossiblyNullArrayOffset in Mapper::getFieldMap

Add explicit null check on the result of fixFieldName before using it
as an array offset, satisfying Psalm's type narrowing requirements.

Author: byjg

src/Mapper.php

@@ -334,7 +334,7 @@ public function getFieldMap(?string $property = null): array|FieldMapping|null
 
         $property = $this->fixFieldName($property);
 
-        if (!isset($this->fieldMap[$property])) {
+        if ($property === null || !isset($this->fieldMap[$property])) {
             return null;
         }