fix: Use ".value()" instead of "->" when with STObject::Proxy objects

- Turns out that "Proxy::operator->" is not a safe substitute for
  "Proxy::value()." if the field is not required. The implementation
  is different such that "operator->" will return a null ptr if the
  field is not present. This includes default fields with a value of
  zero!

include/xrpl/protocol/STObject.h

@@ -482,6 +482,8 @@ public:
     value_type
     operator*() const;
 
+    /// Do not use operator->() unless the field is required, or you've checked
+    /// that it's set.
     T const*
     operator->() const;
 
@@ -739,6 +741,8 @@ STObject::Proxy<T>::operator*() const -> value_type
     return this->value();
 }
 
+/// Do not use operator->() unless the field is required, or you've checked that
+/// it's set.
 template <class T>
 T const*
 STObject::Proxy<T>::operator->() const