SVG: handle empty path data like a path without d - #2144

A <path d=""> fails with ValueError: Path does not start with move item. SVG says empty path data disables rendering of the path; it is not an error.

This comes from real tools: PyMuPDF's Page.get_svg_image() writes <path d=""/> for empty clips (its test output tests/test_4846_out.svg fails to load in fpdf2), and so do ezdxf's documentation drawings.

parse_path() already rejects path data that does not start with a moveto, with its own error, so the check in svg_path_converter() was only reached for empty data, where first_is_move stays None. It now raises only when first_is_move is False, so an empty d gives the same empty path as a <path> without d.

Added test_path_empty_data ("" and whitespace) and test_path_not_starting_with_move, which checks that invalid path data still raises; the first fails on master. pytest test/svg test/drawing, black, pylint and mypy pass