Fix crash in make_fake_toas when add_correlated_noise=True with no correlated-noise component

[Rome-1]

pint.simulation.make_fake_toas(..., add_correlated_noise=True) crashes with a TypeError/AttributeError (object of type 'NoneType' has no len() / 'NoneType' object has no attribute 'shape') when the timing model has no correlated-noise component, instead of either no-opping or raising a clear error.

The cause is that TimingModel.noise_model_designmatrix and TimingModel.noise_model_basis_weight both return None when the model has no correlated-noise component (no PLRedNoise, PLDMNoise, EcorrNoise, etc.), and make_fake_toas dereferences those return values without a guard.

The fix adds a guard around the None case in make_fake_toas: if either matrix is None, it logs a warning and skips the correlated-noise term instead of crashing. This mirrors how TimingModel.full_designmatrix/full_wideband_designmatrix already treat a None noise design matrix elsewhere in the same module, and follows the existing log.warning-and-fall-back convention already used twice in simulation.py for other unexpected-but-non-fatal inputs.

Reproducer (crashes before this fix, runs cleanly after):

from pint.models import get_model
import pint.simulation, pint, pathlib
ex = pathlib.Path(pint.__file__).parent / "data" / "examples"
m = get_model(str(ex / "J0740+6620.FCP+21.wb.DMX3.0.par"))
pint.simulation.make_fake_toas_fromtim(
    str(ex / "J0740+6620.FCP+21.wb.tim"), model=m,
    add_noise=True, add_correlated_noise=True)

Added a regression test, test_simulate_corrnoise_no_noise_components in tests/test_fake_toas.py, that calls make_fake_toas_uniform(..., add_correlated_noise=True) on a model with no correlated-noise component; it fails on master and passes with this change.