Don't clip if path is undefined in SVG back-end

[janpe2]

Fixes #10846

The PDF in the issue uses an empty path as the clipping path:

q
    W*
    n
Q

Because of this, display/svg.js attempts to call function cloneNode of an undefined object.

[Snuffleupagus]

/botio-windows lint
/botio-linux preview

[pdfjsbot]

From: Bot.io (Windows)

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Received

Command cmd_lint from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.215.176.217:8877/8282dcc3603cddf/output.txt

[pdfjsbot]

From: Bot.io (Linux m4)

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Received

Command cmd_preview from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.67.70.0:8877/61539549d60d5b5/output.txt

[pdfjsbot]

From: Bot.io (Linux m4)

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Success

Full output at http://54.67.70.0:8877/61539549d60d5b5/output.txt

Total script time: 1.88 mins

Published

• Viewer: http://54.67.70.0:8877/61539549d60d5b5/web/viewer.html

[pdfjsbot]

From: Bot.io (Windows)

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Success

Full output at http://54.215.176.217:8877/8282dcc3603cddf/output.txt

Total script time: 2.85 mins

• Lint: Passed

[timvandermeij]

Thank you!