fix(thermal): change validation on empty matrices

antarest/study/business/areas/thermal_management.py

@@ -375,7 +375,7 @@ def validate_series(self, study: Study, area_id: str, cluster_id: str) -> bool:
             matrix = self.storage_service.get_storage(study).get(study, matrix_path)
             matrix_data = matrix["data"]
             matrix_length = len(matrix_data)
-            if matrix_length > 0 and matrix_length != 8760:
+            if matrix_data != [[]] and matrix_length != 8760:
                 raise IncoherenceBetweenMatricesLength(
                     f"The matrix {Path(matrix_path).name} should have 8760 rows, currently: {matrix_length}"
                 )