LibWeb: Fail CSS color parse for "rgba(123, 123, 123, "

This matches the behavior of other browsers and fixes a WPT test.

(cherry picked from commit 902586a21de3ef8335e447053d82057c4c927d72;
amended expected output because serenity does not yet have
LadybirdBrowser/ladybird#1603)

Tests/LibWeb/Text/expected/canvas/fillStyle-bogus-rgba-color.txt

@@ -0,0 +1,2 @@
+  #008000
+#008000

Tests/LibWeb/Text/input/canvas/fillStyle-bogus-rgba-color.html

@@ -0,0 +1,12 @@
+<script src="../include.js"></script>
+<canvas id="c" width=300 height=300></canvas>
+<script>
+    test(() => {
+        let x = c.getContext("2d");
+        x.fillStyle = 'green';
+        println(x.fillStyle);
+        x.fillStyle = 'rgba(255, 0, 0, ';
+        println(x.fillStyle);
+        c.remove();
+    });
+</script>

Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -2851,6 +2851,9 @@ RefPtr<CSSStyleValue> Parser::parse_rgb_color_value(TokenStream<ComponentValue>&
 
             alpha = parse_number_percentage_value(inner_tokens);
 
+            if (!alpha)
+                return {};
+
             inner_tokens.skip_whitespace();
 
             if (inner_tokens.has_next_token())