Update column.py closes #63

pandas_schema/column.py

@@ -24,4 +24,9 @@ def validate(self, series: pd.Series) -> typing.List[ValidationWarning]:
         :param series: A pandas Series to validate
         :return: An iterable of ValidationError instances generated by the validation
         """
-        return [error for validation in self.validations for error in validation.get_errors(series, self)]
+
+        # skip validation if column contains no validation objects. Used for columns that require no validation but must be present in DataFrame
+        if self.validations == []:
+            return []
+        else:
+            return [error for validation in self.validations for error in validation.get_errors(series, self)]