Bug report

See title. The new metadata support for SVG backend does not seem to honor a None Date metadata.

Code for reproduction

plot([1, 2], [2, 3])
savefig("a.svg", metadata={'Date': None})

Actual outcome

The output svg file contains the Date metedata

Expected outcome

As the doc says

Values have been predefined for 'Creator', 'Date', 'Format', and 'Type'. They can be removed by setting them to None.

Setting the other three to None indeed works but not for Date.

Matplotlib version

• Operating system: ArchLinux
• Matplotlib version: 3.3.0
• Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg
• Python version: 3.8.3
• Jupyter version (if applicable): (core: 4.6.3) not applicable

Installed from ArchLinux repo.