fix: fallback to X11 capturer if pipewire fails on Wayland

[VerteDinde]

Description of Change

Closes #37463

Desktop Capturer behaves inconsistently on Wayland. We use PipeWire within WebRTC on Wayland to gather desktop window & screen capture sources. However, PipeWire does not always successfully start, and beginning with this CL, we no longer fall back to the X11 capturer, even if it is enabled: https://webrtc-review.googlesource.com/c/src/+/279163

Currently if neither PipeWire or X11 can be used, we return a nullptr in place of the capturer sources, crashing the application.

This PR does two things:

1. Reinstates the previous fallback behavior in WebRTC, so that if PipeWire fails on Wayland, we can still attempt to gather capture sources, rather than simply failing. I'll submit this as a patch for upstream consideration as well.
2. Ensures that a lower level capturer of some kind exists before trying to create one within Electron's DesktopCapturer::StartHandling method. (Thanks @ckerr!)

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant documentation, tutorials, templates and examples are changed or added
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Fixed a crash on capturing sources when using desktopCapturer API on Wayland.

[release-clerk]

Release Notes Persisted

Fixed a crash on capturing sources when using desktopCapturer API on Wayland.

[trop]

I have automatically backported this PR to "23-x-y", please check out #37526

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I have automatically backported this PR to "24-x-y", please check out #37527