Fixed write_usage producing empty output when args is empty.

[tejasae-afk]

Fixes #3360.

When HelpFormatter.write_usage is called with no args (or an empty string), the usage line was silently dropped. The call to wrap_text("", ...) returned an empty string, so only the trailing \n was written to the buffer.

Root cause: wrap_text with an empty text string returns "", writing nothing. The usage_prefix (e.g. "Usage: program ") was only passed as the initial_indent to wrap_text, not written directly.

Fix: Short-circuit when args is empty — write usage_prefix (stripped of its trailing space) directly and skip wrap_text entirely.

Before:

import click
f = click.HelpFormatter()
f.write_usage("program")
print(repr(f.getvalue()))
# '\n'  ← empty line, usage line missing

After:

print(repr(f.getvalue()))
# 'Usage: program\n'  ← correct

Test added in tests/test_formatting.py. Full suite: 1494 passed, 24 skipped.