SVG: draw nothing for a <circle> without r or a <polyline> or <polygon> without points - #2168

A <circle> without r, and a <polyline> or <polygon> without points, fail with a KeyError. In the SVG specification the initial value of r is 0, which disables rendering, and a missing points is an empty list of points, so these elements draw nothing. fpdf2 already does that for an <ellipse> without rx and ry, and for points="".

A circle without r comes up with SMIL animations, where the radius is only set by an <animate attributeName="r" .../> child, for example a pulsing ring:

<circle cx="260" cy="255" fill="none" stroke="#0071e3" stroke-width="5">
  <animate attributeName="r" values="113;119;113" dur="2.5s" repeatCount="indefinite"/>
</circle>

circle() now reads a missing r as 0 and returns an empty path for r="0", like ellipse(); polyline() and polygon() read a missing points as "". Before, r="0" drew a degenerate circle, which can show as a dot with a round-capped stroke.

The circle no r, polyline no points and polygon no points cases of test_svg_shape_tags expected the KeyError; they now expect no path, and a new circle r 0 case is added. All four fail on master. The SVG tests, black and pylint pass