fix: guard against undefined testFiles when findTests throws

[tsushanth]

What

When globs.findTests rejects (bad glob config, permission error, IO failure), the catch block in Api#run stores the error in setupOrGlobError but leaves testFiles as undefined. The very next statement builds selectionInsights:

const selectionInsights = {
  ...
  testFileCount: testFiles.length,   // ← TypeError: Cannot read properties of undefined
  ...
};

This crashes before setupOrGlobError is ever re-thrown, so the original error is swallowed and the user sees a confusing TypeError instead.

Fix

Use optional chaining so the property access is safe when testFiles is undefined:

testFileCount: testFiles?.length ?? 0,

Reproduction

Configure ava with a files glob that causes findTests to throw (e.g., pass an invalid pattern that triggers the internal validation error), or simulate an IO failure in globs.findTests. The run crashes with:

TypeError: Cannot read properties of undefined (reading 'length')
    at Api.run (lib/api.js:...)

instead of surfacing the underlying glob/config error.