Desktop: Fixes #10382: Fix default values for plugin settings stored in settings.json not applied

[personalizedrefrigerator]

Summary

This pull request fixes an issue where plugin settings saved using SettingStorageType.File did not use the setting's specified default value.

Fixes #10382.

Notes

This pull request makes Setting.loadOne return null when a file setting has no saved value. This is consistent with the behavior for database settings. Previously, loadOne would return an object for a file setting, even when that setting had no saved value. This would cause the setting to be loaded into the cache (without using its default value):

joplin/packages/lib/models/Setting.ts

Lines 2014 to 2023 in 443e04b

	const valueRow = await this.loadOne(key);
	if (valueRow) {
	// Remove any duplicate copies of the setting -- if multiple items in cache_
	// have the same key, we may encounter unique key errors while saving to the
	// database.
	this.cache_ = this.cache_.filter(setting => setting.key !== key);
	this.cache_.push({
	key: key,
	value: this.formatValue(key, valueRow.value),

Testing plan

This pull request has an automated test. It has also been tested manually by:

1. Installing the CodeMirror 6 settings plugin
   • Remove settings.json if the plugin was previously installed.
2. Restarting Joplin
3. Verifying that the value for the "line numbers" setting is true.