Fix another issue with getTemplateLocals and block params

[NullVoxPopuli]

This one stems from ElementNode not having a parsed tag name -- it's just a string, instead of potentially a PathExpression, like it would/should be in the case of:

<Wrapper as |w|>
    <w.button />
{{! ^ ElementNode
     ^ PathExpression { parts: ['w', 'button' ] }
}}

</Wrapper>

[wycats]

Looks good!

I want to update this code to use ASTv2, which doesn't have any of these issues, but this is fine as an interim measure.