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/rpc/ServerInfo_test.cpp

@@ -85,7 +85,7 @@ admin = 127.0.0.1
                     (git[jss::hash].isString() && git[jss::hash].asString().size() == 40));
                 BEAST_EXPECT(
                     !git.isMember(jss::branch) ||
-                    (git[jss::branch].isString() && git[jss::branch].asString().size() != 0));
+                    (git[jss::branch].isString() && !git[jss::branch].asString().empty()));
             }
         }