alt-text: missing warning for empty alt for <input type="image" alt="" />

[EvgenyOrekhov]

<input type="image" alt="" /> should be considered invalid because <input> is an interactive element, it can't be "decorative", thus it has to have a non-empty alt.

Note: SonarJS does warn in this case, see "Noncompliant code example" here: https://sonarsource.github.io/rspec/#/rspec/S1077/javascript