fix: Shallow copy input dict in _empty_field_to_none validator

Avoid mutating the caller's dict by making a shallow copy before
modifying empty Optional fields.

Co-Authored-By: Rahul Kaushal <kaushalrahul15@gmail.com>

Author: msto

fgmetric/metric.py

@@ -82,6 +82,8 @@ def _empty_field_to_none(cls, data: Any) -> Any:
             # short circuit
             return data
 
+        data = dict(data)
+
         for field, value in data.items():
             info = cls.model_fields.get(field)
             if info is None: