Add nothing case for theme

[pepijndevos]

Description

When calling theme(scene, :key) in a recipe, if scene is nothing it seems to look up the default theme.
But theme(scene) has no such fallback.

Type of change

Delete options that do not apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Added an entry in NEWS.md (for new features and breaking changes)
• Added or changed relevant sections in the documentation
• Added unit tests for new algorithms, conversion methods, etc.
• Added reference image tests for new plotting functions, recipes, visual options, etc.

[SimonDanisch]

Hm, the thing is, CURRENT_DEFAULT_THEME shouldn't be directly handed to the user for mutation.
Although, set_theme! also mutates it, but I'm not sure I want more ways to mutate the default_theme.
What exactly do you want to do with theme()?

[pepijndevos]

Get an attribute that may or may not exist. Apparently a recipe gets called with a nothing scene sometimes, so if you write get(theme(scene), key, default) instead of theme(scene, key) it'll error

[SimonDanisch]

Ah maybe we should add theme(scene, key; default) for more signatures then...

[pepijndevos]

That would be another option yes. For reference, this is a line of code I currently have:

        xgridcolor = get(something(get(theme(scene), :Axis, nothing), Attributes()), :xgridcolor, RGBAf(0, 0, 0, 0.12)),

[SimonDanisch]

Do you want to implement theme(scene, key; default) in this PR?

[pepijndevos]

Sure after I'm back from holiday :)

[pepijndevos]

Superseded by #3214 since this one is like an one line change that's now out of date