Part extraction bugs

[rowanc1]

There is a duplication bug:

# testing

+++ {"part": "abstract"}

this is duplicated

# testing

## Abstract

this is duplicated

And then an error on a blank block (in the theme in myst-common):

# testing

+++ {"part": "abstract"}

this is duplicated

+++

[fwkoch]

Ok - the first couple examples are happening when there is nothing else in the file except the abstract. In these cases, when extractParts called remove, it was removing the entire tree and therefore failing. That's easy to fix - just add a cascade: false failsafe if the entire tree is removed. (We already do this exact thing here: https://github.com/executablebooks/mystmd/blob/main/packages/myst-transforms/src/frontmatter.ts#L84-L88)

For the last case, we should definitely add a ?: tree.children?.forEach. But also, blocks should always have children defined... There's a line here where we explicitly remove empty children: https://github.com/executablebooks/mystmd/blob/main/packages/myst-parser/src/tokensToMyst.ts#L577 - I think leaving the empty array is better?

I made these changes here: #1055

For this to fully take effect, myst-theme will need to consume the changes to myst-common

[rowanc1]

Thanks for the PRs, leaving this open til we bring it over to the theme.

[fwkoch]

Version bumps that bring this into the theme are included with this pr: executablebooks/myst-theme#351

[fwkoch]

Fixed in the latest release of themes. You may need to myst clean --templates!