Tests/LibWeb: Verify instantiation behavior of Blob w/empty constructor

This verifies that a Blob instantiated with an empty constructor creates
a Blob with Blob.size equal to 0 and Blob.type equal to "".

Tests/LibWeb/Text/expected/FileAPI/Blob-empty-constructor.txt

@@ -0,0 +1 @@
+PASS

Tests/LibWeb/Text/input/FileAPI/Blob-empty-constructor.html

@@ -0,0 +1,10 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const blob = new Blob;
+        if (blob.size === 0 && blob.type === "")
+            println("PASS");
+        else
+            println("FAIL");
+    });
+</script>