Clear Imm32InputMethod.Composition after receiving cancel message (WM_IME_COMPOSITION with lParam set to 0)

[MineCake147E]

What does the pull request do?

This PR clears Imm32InputMethod.Composition when AppWndProc receives WM_IME_COMPOSITION message with lParam set to 0, which apparently means that the IME canceled the composition entirely.

What is the current behavior?

HandleComposition does nothing if lParam were 0, which causes #18021 by HandleCompositionEnd erroneously executes _parent.Input(e); with uncleared Composition.

What is the updated/expected behavior with this PR?

HandleComposition now clears Composition if lParam were 0. HandleCompositionEnd now sees the empty Composition after IME cancelling the composition.

How was the solution implemented (if it's not obvious)?

if (flags == 0)
{
    CompositionChanged("");
}

Checklist

• Added unit tests (I don't think it's possible since it needs IME installed in testing environment)
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

Fixes #18021

[Gillibald]

Looks good. Thank you for your contribution.

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054487-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.