jsx-no-target-blank crashes on empty `rel` attribute

[not-an-aardvark]

Using eslint@4.0.0 and eslint-plugin-react@7.1.0, the jsx-no-target-blank rule crashes on the following code:

<a href="https://example.com" target="_blank" rel />

This can be reproduced with:

echo '<a href="https://example.com" target="_blank" rel />' | node_modules/.bin/eslint --stdin --no-eslintrc --plugin=react --parser-options='ecmaFeatures: {jsx: true}' --rule 'react/jsx-no-target-blank:error'

This is a very minor issue, since I don't know of a good use case for having an empty rel attribute. I only noticed it because a crash notification appeared in my editor.