Saving annotations as CSV results in wrong orig_time

Describe the bug

When creating annotations with orig_time=None and saving them to a *.csv file using the save function, orig_time=None is ignored and replaced by 1970-01-01 00:00:00. Thus, when importing the annotations again, the orig_time of the imported annotations differs from the original ones. This seems odd to me and not intended, because the issue does not occur when saving the annotations to *.txt file

Steps to reproduce

Here is a minimal working example:

# Create annotation and save it to CSV file
annotations = mne.Annotations(onset=[2.5], duration=[5], description=["Test"], orig_time=None)
annotations.save("annotation.csv")

# Read annotation from file again
read_annotations = mne.read_annotations("annotation.csv")

# Output orig_time
print(read_annotations[0]['orig_time']) #Outputs 1970-01-01 00:00:02.500000+00:00 instead of None

Expected results

I expected the output to be None. This is even the result when using a *.txt file instead of a *.csv file.

Actual results

1970-01-01 00:00:02.500000+00:00
In the annotations dict, the orig_time is stored as ('orig_time', datetime.datetime(1970, 1, 1, 0, 0, 2, 500000, tzinfo=datetime.timezone.utc))

Start by tracing the Annotations.save and read_annotations entry points for CSV handling, using the reproduction in the issue to compare CSV and TXT behavior. Confirm that an annotation saved with orig_time=None is imported with orig_time=None, and add or update coverage for this round trip.