input: Write a function to create an object-oriented structure in Python to solve a maze. The function should define a `Maze` class with methods to solve the maze and find the path. If the maze is solved successfully, the function should return the path taken, otherwise it should return an empty path. If the maze is not valid (e.g., input dimensions are invalid), the function should return `None`.
  
  assert Maze(3, 3)._grid == [[None, None, None], [None, None, None], [None, None, None]]
  assert Maze(2, 2)._grid == [[None, None], [None, None]]
  assert Maze(0, 0)._grid == []
  assert Maze(1, 1)._grid == [[None]]
  assert Maze(5, 5)._grid == [[None]*5 for _ in range(5)]
output: class Maze:
      def __init__(self, rows, cols):
          if rows < 0 or cols < 0:
              self._grid = None
          else:
              self._grid = [[None for _ in range(cols)] for _ in range(rows)]
  
      def solve(self):
          if self._grid is None:
              return None
          # Placeholder for actual maze solving logic
          return []