[tools] Store list of trampolines even if it's empty. Fixes #18570.

[rolfbjarne]

This is a bit non-obvious, but we later use the presence of such a stored list
to determine if we need to generate the Registrar type in the assembly.

The problem is that we need to generate the Registrar type even if no
trampolines were created, because we use it for numerous other purposes as
well (type lookup for instance).

Fixes #18570.

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NET (empty diffs)

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Legacy Xamarin (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• watchOS: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)

.NET (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• MacCatalyst: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)
• Microsoft.iOS vs Microsoft.MacCatalyst: vsdrops gist

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• tvOS: vsdrops gist
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