fix: handle null inline style declarations in style attribute comparers#59
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StyleAttributeComparer and OrderingStyleAttributeComparer passed the result of IElement.GetStyle() straight into CompareCssStyleDeclarations, which dereferences it. GetStyle() returns null when an element has no inline CSS style declaration to parse — a non-HTML (SVG/MathML) element, or any element when the browsing context has no ICssParser registered (e.g. consumers that don't call .WithCss(), such as bUnit). Comparing a styled element in that situation threw a NullReferenceException. Both comparers now fall back to comparing the raw style attribute value when either declaration is null, so such elements compare by value instead of throwing. Adds regression tests that build the comparison in a CSS-less browsing context, since the shared test fixture enables CSS and so cannot reproduce the issue.
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StyleAttributeComparer and OrderingStyleAttributeComparer passed the result of IElement.GetStyle() straight into CompareCssStyleDeclarations, which dereferences it. GetStyle() returns null when an element has no inline CSS style declaration to parse — a non-HTML (SVG/MathML) element, or any element when the browsing context has no ICssParser registered (e.g. consumers that don't call .WithCss(), such as bUnit). Comparing a styled element in that situation threw a NullReferenceException.
Both comparers now fall back to comparing the raw style attribute value when either declaration is null, so such elements compare by value instead of throwing.
Adds regression tests that build the comparison in a CSS-less browsing context, since the shared test fixture enables CSS and so cannot reproduce the issue.