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/protocol/STAccount_test.cpp

@@ -12,7 +12,7 @@ struct STAccount_test : public beast::unit_test::suite
             // Test default constructor.
             STAccount const defaultAcct;
             BEAST_EXPECT(defaultAcct.getSType() == STI_ACCOUNT);
-            BEAST_EXPECT(defaultAcct.getText() == "");
+            BEAST_EXPECT(defaultAcct.getText().empty());
             BEAST_EXPECT(defaultAcct.isDefault() == true);
             BEAST_EXPECT(defaultAcct.value() == AccountID{});
             {
@@ -38,7 +38,7 @@ struct STAccount_test : public beast::unit_test::suite
             // Test constructor from SField.
             STAccount const sfAcct{sfAccount};
             BEAST_EXPECT(sfAcct.getSType() == STI_ACCOUNT);
-            BEAST_EXPECT(sfAcct.getText() == "");
+            BEAST_EXPECT(sfAcct.getText().empty());
             BEAST_EXPECT(sfAcct.isDefault());
             BEAST_EXPECT(sfAcct.value() == AccountID{});
             BEAST_EXPECT(sfAcct.isEquivalent(defaultAcct));