Fix HelpFormatter.write_usage with no args

[ibondarenko1]

Closes #3360.

When HelpFormatter.write_usage is called without an args value, the resulting output is a single blank line instead of the expected Usage: <prog> line. This happens because wrap_text("", initial_indent=usage_prefix, ...) ends up calling textwrap.TextWrapper.fill(""), which returns "" and discards initial_indent.

This short-circuits the empty-args path and writes the usage prefix directly. The trailing space added between prog and args is rstrip-ped before the newline so the output is exactly Usage: <prog> .

Reproducer (from #3360)

import click
f = click.HelpFormatter()
f.write_usage("program")
print(f.getvalue())

• Before: empty line
• After: Usage: program

Tests

Added three tests in tests/test_formatting.py:

• test_help_formatter_write_usage_no_args — default prefix.
• test_help_formatter_write_usage_no_args_custom_prefix — custom prefix preserved.
• test_help_formatter_write_usage_no_args_indented — indent applied via the prefix:>{indent} format spec is preserved (rstrip only strips trailing whitespace).

pytest -x -q passes (1389 passed, 73 skipped, 1 xfailed).