Resolve handling of {}**0 in LinearRepn/QuadraticRepn

[jsiirola]

Fixes # .

Summary/Motivation:

This resolves an error we were seeing in the performance tests where x**0 was being categorized as a nonlinear expression instead of resolving it to the constant 1.

Changes proposed in this PR:

• Compile {}**0 to 1 in LinearRepnVisitor / QuadraticRepnVisitor
• Update tests to cover this case

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