fix: add Non-empty judgment

[leafstark]

I added a non-empty judgment to the variable that may cause a TypeError（
TypeError: Cannot read property 'references' of null
） in _iterateVariableReferences
for (const reference of variable.references) {

[leafstark]

@mysticatea would you please have a look at this PR? While running ESLint, TypeError: Cannot read property 'references' of null occurs when users use eslint-plugin-vue and unplugin-vue2-script-setup. Thanks in advance!

[ota-meshi]

Hi @leafstark.

I think it's an eslint-plugin-vue issue, not an eslint-utils issue.
Also, the eslint-plugin-vue issue has probably been fixed.
vuejs/eslint-plugin-vue#1690

If you still can't solve it, please open the issue in eslint-plugin-vue. In that case, please also provide a repository to reproduce the issue.