NLv2: only raise exception for empty models in the legacy API

[jsiirola]

Fixes #3131 .

Summary/Motivation:

This updates the NLv2 writer to only raise an exception for empty models (no variables in either objectives or constraints) when it is called using the legacy "call" interface. The new recommended interface will not raise an exception and instead rely on the caller (the solver interface) to inspect the returned list of variables and do the correct thing when that list is empty.

Changes proposed in this PR:

• Move the exception for an empty model from the core writer method and into the logacy __call__ interface.

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[michaelbynum]

Thanks, @jsiirola!