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/libxrpl/protocol/STVar.cpp

@@ -61,9 +61,13 @@ STVar::operator=(STVar const& rhs)
     {
         destroy();
         if (rhs.p_)
+        {
             p_ = rhs.p_->copy(max_size, &d_);
+        }
         else
+        {
             p_ = nullptr;
+        }
     }
 
     return *this;
@@ -116,9 +120,13 @@ void
 STVar::destroy()
 {
     if (on_heap())
+    {
         delete p_;
+    }
     else
+    {
         p_->~STBase();
+    }
 
     p_ = nullptr;
 }