Browser+WebDriver: Set the initial URL to about:blank from WebDriver

We already do this for headless-browser. There's no need to open any URL
other than about:blank when starting a WebDriver session. We should also
do this from WebDriver code, rather than in special logic in Browser's
main.cpp.

Ladybird/WebDriver/main.cpp

@@ -36,6 +36,7 @@ static ErrorOr<pid_t> launch_browser(DeprecatedString const& socket_path)
         Array {
             "--webdriver-content-path",
             socket_path.characters(),
+            "about:blank",
         });
 }