In the lobby of a downtown Manhattan tower, Nathan Berman pauses to admire the marble walls and terrazzo floors. A real estate developer, he has a taste for old-world detail, like the fur lining in his navy overcoat, accented with a polka-dot scarf. It’s rush hour in the heart of Wall Street, and Berman’s at its white-hot center, 55 Broad St., former offices of Goldman Sachs Group Inc. But, like many buildings in the age of working from home, this onetime hub of capitalism is largely empty. Many of Berman’s rivals would be discouraged. He’s thrilled.