fix(language-service): fix caching of semantic diagnostics

[remcohaszing]

This fixes a caching issue regarding semanric diagnostics. I don’t fully understand why. Returning an empty array from https://github.com/mdx-js/mdx-analyzer/blob/f6a43853d5ae0509b93001fdf6f58e1466106605/packages/language-service/lib/service-plugin.js#L163 fixed the issue, so I figured this was an upstream problem in Volar.

Refs mdx-js/mdx-analyzer#400

[johnsoncodehk]

LGTM, thanks!

The reason is that when the service returns undefined result, the following lines cannot replace the response cache.

volar.js/packages/language-service/lib/features/provideDiagnostics.ts

Lines 284 to 287 in 74b3600

	if (result) {
	cache.errors = result;
	cache.snapshot = sourceFile?.snapshot;
	}