LibWeb/CSS: Fix linear-gradient single color-stop usage

The Web::CSS::Parser's GradientParsing ignores color-stops if
it is only a single one. This change allows to have color-stops
with double positions against a single color.

Further, also allows for `linear-gradient(black)` and similar
other gradient functions

Libraries/LibWeb/CSS/StyleValues/LinearGradientStyleValue.h

@@ -40,7 +40,7 @@ public:
 
     static ValueComparingNonnullRefPtr<LinearGradientStyleValue> create(GradientDirection direction, Vector<LinearColorStopListElement> color_stop_list, GradientType type, GradientRepeating repeating)
     {
-        VERIFY(color_stop_list.size() >= 2);
+        VERIFY(!color_stop_list.is_empty());
         return adopt_ref(*new (nothrow) LinearGradientStyleValue(direction, move(color_stop_list), type, repeating));
     }