ReadGate PyModel not working with target_cost=None

[AlessandroPierro]

Describe the bug
The ReadGate doesn't work with timeout=None when running on CPU.

To reproduce current behavior
Steps to reproduce the behavior:

1. When I run this code (add code or minimum test case) ...

from lava.lib.optimization.problems.problems import QUBO
from lava.lib.optimization.solvers.generic.solver import OptimizationSolver
import numpy as np

Q = np.array([[11, -5, 2, 0, -6], 
              [-5, -13, -1, -3, 10], 
              [2, -1, 0, 2, 10], 
              [0, -3, 2, 2, -1], 
              [-6, 10, 10, -1, -3]])

problem = QUBO(Q)
solver = OptimizationSolver(problem)

solution = solver.solve(timeout=50,
    target_cost=None
)

2. I get this error ...

TypeError: '<=' not supported between instances of 'int' and 'NoneType'

Expected behavior
The OptimizationSolver should run until the timeout is reached.

Environment (please complete the following information):

• Device: Intel cloud
• OS: Linux
• Lava version 0.6.1