[6.x] Fix error caused by empty globals

[duncanmcclean]

This pull request fixes an issue where the v5 to v6 update script doesn't account for globals with empty data.

In v5, it was possible to have empty data in globals:

# content/globals/socials.yaml
title: Socials
data:

Once the updater had run, the result would be:

# content/globals/default/socials.yaml
null

Which then throws an error:

array_key_exists(): Argument #2 ($array) must be of type array, null given

This was happening because Arr::get($contents, 'data', []) returns null when the data key exists but has a null value (the default [] is only used when the key doesn't exist). The YAML::dump(null) call then produces the literal string null.

This PR fixes it by:

• Using Arr::get($contents, 'data') ?? [] to properly handle null values
• Adding defensive null handling in GlobalVariablesStore so it gracefully handles YAML files containing null

Fixes #14095