Update hook in localgov_subsites_paragraphs new install file fails to check for field existence

[ctorgalson]

The update hook creates two new fields for the localgov_paragraphs module, but fails to check if they're already present.

This means a workflow like that shown below will fail on the final step because drush updb creates the fields, so the subsequent drush cim step finds the fields and config already exist.

• composer install latest on dev
• drush deploy on dev
• export config on dev
• capture new config in repo
• deploy updated codebase to prod/stage/etc
• drush deploy on prod/stage/etc

[ctorgalson]

In fact, testing shows that this is only a problem if drush cim runs before drush updb (contrary to drush deploy's order).

Nevertheless, it has happened, so the update hook should check before creating new fields and config.

[louwrense]

@ekes I've confirmed with @tran-bigbluedoor that the PR above is working. Running this updated hook on a project where those fields are already in config no longer causes the import to fail. So all good from our side, thank you.