fix(material-experimental/mdc-select): fix floating label behavior

[mmalerba]

No description provided.

[crisbeto]

I think the changes are fine, but I implemented it this way on purpose. I always found it weird that the label floats, even though we have nothing to show in its place. It wasn't as noticeable on the old select since the overlay was on top of the label most of the time, but that's not the case for the MDC one.

[mmalerba]

I'm just going for consistency with the MDC one. This is what the MDC one does, though we could bring up with them whether this is what it should do.

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