[BUG] segfault when writing MPS with an empty row

[mlubin]

The following code segfaults on main and 26.6:

from cuopt.linear_programming import Problem
from cuopt.linear_programming.problem import LinearExpression

problem = Problem("empty-row")

x = problem.addVariable(
    name="x",
    lb=0.0,
    ub=1.0,
)

# A valid constant-only constraint: 0 <= 1.
empty_expression = LinearExpression(
    vars=[],
    coefficients=[],
    constant=0.0,
)
problem.addConstraint(empty_expression <= 1.0, name="empty")

problem.setObjective(x)
problem.writeMPS("empty-row.mps")