Fix HelpFormatter.write_usage with empty args producing no output

[driphtyio]

Fixes #3360

Problem

When write_usage is called with no args (the default ""), wrap_text("") produces an empty string, so no usage line is printed at all.

Reproduction

import click
f = click.HelpFormatter()
f.write_usage("program")
print(f.getvalue())  # empty line instead of "Usage: program\n"

Fix

Added an early return that writes prefix + prog directly when args is empty or falsy. This avoids passing an empty string to wrap_text.