Fix crash in `canvas_item_add_polyline` when passing more points than colors

When `p_points.size() > p_colors.size()`, it crashed with invalid
array access to `p_colors`. Also, when `p_colors` was an empty
`Vector` it crashed due a missing `else` checking the `size`
condition, as the code handling that special case exists.

This PR fixes the missing `else` for `p_colors.size == 0` and,
following the `canvas_item_add_multiline` spirit, it only uses the
first color for the whole polyline if points and colors differ in
size.

Fix #17621.

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