refactor: Replace isTrue with get_bool

This makes the code more robust, see previous commit.

In general replacing isTrue with get_bool is not equivalent because
get_bool can throw exceptions, but in this case, exceptions won't happen
because of RPCTypeCheck() and isNull() checks in the preceding code.

src/rpc/rawtransaction.cpp

@@ -304,7 +304,7 @@ static RPCHelpMan createrawtransaction()
 
     std::optional<bool> rbf;
     if (!request.params[3].isNull()) {
-        rbf = request.params[3].isTrue();
+        rbf = request.params[3].get_bool();
     }
     CMutableTransaction rawTx = ConstructTransaction(request.params[0], request.params[1], request.params[2], rbf);
 
@@ -1449,7 +1449,7 @@ static RPCHelpMan createpsbt()
 
     std::optional<bool> rbf;
     if (!request.params[3].isNull()) {
-        rbf = request.params[3].isTrue();
+        rbf = request.params[3].get_bool();
     }
     CMutableTransaction rawTx = ConstructTransaction(request.params[0], request.params[1], request.params[2], rbf);