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/basics/Buffer_test.cpp

@@ -12,7 +12,7 @@ struct Buffer_test : beast::unit_test::suite
     bool
     sane(Buffer const& b) const
     {
-        if (b.size() == 0)
+        if (b.empty())
             return b.data() == nullptr;
 
         return b.data() != nullptr;
@@ -241,13 +241,13 @@ struct Buffer_test : beast::unit_test::suite
                 // Try to clear:
                 x.clear();
                 BEAST_EXPECT(sane(x));
-                BEAST_EXPECT(x.size() == 0);
+                BEAST_EXPECT(x.empty());
                 BEAST_EXPECT(x.data() == nullptr);
 
                 // Try to clear again:
                 x.clear();
                 BEAST_EXPECT(sane(x));
-                BEAST_EXPECT(x.size() == 0);
+                BEAST_EXPECT(x.empty());
                 BEAST_EXPECT(x.data() == nullptr);
             };