Use data type configuration to determine default value for empty toggle and slider property values

[AndyButland]

Prerequisites

• I have added steps to test this contribution in the description below

Fixes #17833, tracked under AB 45702 (internal HQ tracker)

Also Fixes #17099, tracked under AB 47510

Description

Reported issue: For toggle properties set to the initial value of true, this is not working. The value is stored as false even though it is displayed as “on” in the UI.

Background: If the user does not interact with a Property Editor, we do not store anything.
This is how the Client works as of v.14, and this plays well with the fact that we do not render Property Editors that the user haven't navigated to, meaning we do not guarantee that the Property Editor UI would be available to set an initial(default) value.
In other words, Default Values should be set by the server.

Reproduction:

• Create two data types based on the "Toggle" property editor named "Toggle 1" and "Toggle 2".
• For one set the "Initial state" value to "on", for the other leave it "off".
• Add the two data types to a document type and render in a template like the following:

@using Umbraco.Cms.Web.Common.PublishedModels;
@inherits Umbraco.Cms.Web.Common.Views.UmbracoViewPage<ContentModels.HomePage>
@using ContentModels = Umbraco.Cms.Web.Common.PublishedModels;
@{
  Layout = null;
}

<ul>
  <li>Setting 1: @Model.Toggle1</li>
  <li>Setting 2: @Model.Toggle2</li>
</ul>

• Create a content item based on the document type, note that one toggle will be on and one off as expected.
• Save and publish the document type without touching the toggles.
• Render the page and note that both toggle values are false.

With this PR I've added a check for null values in the property value converter, when, if found, the default value from the data type configuration will be used.

To Test:

• Follow the reproduction steps described and note that with the PR applied that the expected values are displayed.
• Edit the content, setting the toggle values explicitly, and confirm you again see the expected values.
• Repeat for the slider.

[nielslyngsoe]

Fixes #17833

[AndyButland]

I found a similar issue raised for the slider property editor, so have fixed that in a similar way as part of this PR.

[nikolajlauridsen]

Looks good, and tests good, I did a very minor cleanup renaming a variable, hope that's okay 🙏