Clarify Identity rules: reject / never return Identity with an empty name

[bernardnormier]

Most of the Ice APIs don't accept or return an Identity with an empty name, with at least one exception: stringToIdentity returns an empty-name Identity (== illegal identity) for the empty string.

I propose to tighten the rules as follows: no Ice API ever accepts or returns an Identity with an empty name.

That's much clearer. Naturally you can still create an Identity with an empty name (that's what the default ctor does). But you can't pass it as an argument successfully to any Ice API.

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