Closes #16

Problem

parseAriaTemplate('') throws "Aria snapshot must be a YAML sequence" because
yaml.parseDocument('') returns contents: null, which is not a YAMLSeq.

Fix

Add an early return for null contents — return an empty fragment node
{ kind: 'role', role: 'fragment' } with no errors. This also simplifies the
existing error path since yamlDoc.contents is guaranteed non-null after the
new guard, removing the ternary fallback for the range.

Context

This unblocks the vitest-side fix for vitest-dev/vitest#10158
(vitest-dev/vitest#10188), which correctly distinguishes empty string from
undefined in domain snapshots but needs ivya to handle empty templates.

Test

Updated the existing empty input test case in test/aria.test.ts to expect
an empty fragment instead of an error.