Support empty strings as the repeat argument (CreatePattern)
According to the third step in the specification [1], createPattern
should let the repetition argument be "repeat" when it is the empty
string.
The code in CanvasRenderingContext2D::CreatePattern did not implement
this step and instead threw a SyntaxError exception when an empty
string was supplied as the repetition argument.
Fixes#9079.
[1] https://html.spec.whatwg.org/multipage/#dom-context-2d-createpattern
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