Fix HelpFormatter.write_usage dropping prefix when args is empty

[bibekmhj]

Fixes #3360.

Tracked this down — when args="" the function calls wrap_text("", ..., initial_indent=usage_prefix, ...), but textwrap.fill of an empty string returns "" without emitting the initial_indent. So the "Usage: program " prefix gets dropped on the floor and only the trailing newline makes it through. Empty line, just like the issue.

So the fix just writes the prefix directly when there are no args, and only goes through wrap_text when there's something to wrap. The else-branch (prefix too long) doesn't hit this because it already writes usage_prefix before the wrap call.

Added a regression test using the reporter's repro.