mac-capture: Avoid inadvertent capture of screen content

[jcm93]

Description

Adds null or placeholder values as the first elements of macOS Screen Capture content lists, and removes the automatic selection of the user's primary display for capture.

Motivation and Context

This change is designed to end any inadvertent capture of the user's screen content which was not expressly selected for capture by the user.

When opening a source properties window in OBS, property lists by default typically select the first element. If the source is an SCK source and the list represents running applications, windows, or displays, the first enumerated content in the list would be selected on opening the properties window, causing that content to immediately begin streaming. This could be a significant privacy concern.

Related: #11233

How Has This Been Tested?

Tested on Apple Silicon, macOS 15.7.1.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.