Fix getScriptSnapshot for file with empty content

[nanokind]

After applying env.updateFile to reset a file's content to empty, then use any env.languageService.* method, it would throw an error message stating "Could not find source file: 'index.ts'"

import {
  createDefaultMapFromCDN,
  createSystem,
  createVirtualTypeScriptEnvironment,
} from '@typescript/vfs';
import ts from 'typescript';

const fsMap = await createDefaultMapFromCDN(
  { target: ts.ScriptTarget.ES2018 },
  '5.7.2',
  false,
  ts
);

const system = createSystem(fsMap);

const compilerOpts = {};
const env = createVirtualTypeScriptEnvironment(system, [], ts, compilerOpts);

env.createFile('/index.ts', 'console.logx()');

env.updateFile('/index.ts', '');

// throws error: Could not find source file: 'index.ts'.
env.languageService.getSemanticDiagnostics('index.ts');

Here is the repro: https://stackblitz.com/edit/vitejs-vite-obbb75xc?file=src%2Findex.ts

This PR fixes this

[nanokind]

@microsoft-github-policy-service agree