refactor: Add simple clang-tidy readability checks (#6556)

This change enables the following clang-tidy checks:
-  readability-avoid-nested-conditional-operator,
-  readability-avoid-return-with-void-value,
-  readability-braces-around-statements,
-  readability-const-return-type,
-  readability-container-contains,
-  readability-container-size-empty,
-  readability-else-after-return,
-  readability-make-member-function-const,
-  readability-redundant-casting,
-  readability-redundant-inline-specifier,
-  readability-redundant-member-init,
-  readability-redundant-string-init,
-  readability-reference-to-constructed-temporary,
-  readability-static-definition

src/test/app/DNS_test.cpp

@@ -15,7 +15,7 @@ class DNS_test : public beast::unit_test::suite
     using endpoint_type = boost::asio::ip::tcp::endpoint;
     using error_code = boost::system::error_code;
     std::weak_ptr<xrpl::detail::Work> work_;
-    endpoint_type lastEndpoint_{};
+    endpoint_type lastEndpoint_;
     parsedURL pUrl_;
     std::string port_;
     jtx::Env env_;
@@ -76,7 +76,7 @@ public:
     parse()
     {
         std::string url = arg();
-        if (url == "")
+        if (url.empty())
             url = "https://vl.ripple.com";
         BEAST_EXPECT(parseUrl(pUrl_, url));
         port_ = pUrl_.port ? std::to_string(*pUrl_.port) : "443";