Implement a nullish coalescing operator

The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is ~null or undefined~ empty, and otherwise returns its left-hand side operand.

Sample usage:
`const port = arg1 ?? 4000;`

Start by locating the TypeScript parser, evaluator, and tests for logical operators; the issue does not name specific files or entry points. Trace how the sample `const port = arg1 ?? 4000;` is parsed and evaluated, then add coverage showing the right-hand side is used for an empty left-hand value and otherwise the left-hand value is preserved.

Written by the indexing model from the issue text.