publish mode allow empty string

[hip3r]

Summary

Allow empty string setting for publishing mode. Issue https://github.com//decaporg/decap-cms/issues/7102

Test plan

Before:

After:

Checklist

Please add a x inside each checkbox:

• [x] I have read the contribution guidelines.

A picture of a cute animal (not mandatory but encouraged)

[privatemaker]

The Docs update looks good to me 👍🏻 the 🐱 picture is 👌🏻 too!

[privatemaker]

@martinjagodic does this look good to you codewise?

[martinjagodic]

@privatemaker thanks for the reminder, this one slipped past me a bit