fix cpp09

[stanX19]

This pull request makes a minor logic fix and a small formatting change in the RPN.cpp file for the Reverse Polish Notation calculator. The main update ensures that the input parsing function correctly validates non-empty input, and there is also a minor reformatting for readability in the compute function.

Input validation improvement:

• Modified RPN::parseInput to return true only if at least one valid token is parsed, preventing empty input from being considered valid.

Code readability:

• Reformatted variable declarations and stack operations in RPN::compute for improved clarity, with no change to logic.