`result_df` does not exist when setting `expand_attribute_column=False`

[SilvanCodes]

As can be seen in the snippet referenced below, the variable result_df does not exist, when expand_attribute_column evaluates to False which renders this argument unusable.

https://github.com/openvax/gtfparse/blob/0432f40185542b1808522573295067e20ba33f34/gtfparse/read_gtf.py#L246C5-L252C60