fix: IEC API change - Meter Reading optional fields (#154)

Author: GuyKh

Diff for: iec_api/models/meter_reading.py

@@ -44,9 +44,9 @@
 class MeterReading(DataClassDictMixin):
     """Meter Reading dataclass."""
 
-    reading: int = field(metadata=field_options(alias="reading"))
-    reading_date: datetime = field(metadata=field_options(alias="readingDate"))
-    serial_number: str = field(metadata=field_options(alias="serialNumber"))
+    reading: Optional[int] = field(default=None, metadata=field_options(alias="reading"))
+    reading_date: Optional[datetime] = field(default=None, metadata=field_options(alias="readingDate"))
+    serial_number: Optional[str] = field(default=None, metadata=field_options(alias="serialNumber"))
     reading_code: Optional[str] = field(default=None, metadata=field_options(alias="readingCode"))
     usage: Optional[str] = field(default=None, metadata=field_options(alias="usage"))