[Fix] Checking nullptr on Node.cpp and ConstantFolding.cpp

model = aidge_core.sequential([
    aidge_core.Conv2D(in_channels=2, out_channels = 2, kernel_dims = [1,1], name = 'conv', stride_dims= [1, 1], dilation_dims = [1, 1], no_bias= False),
    aidge_quantization.Quantizer(scalingFactor = 0.005, clip_min = -128.0, clip_max = 127.0, name="scale")
    
])


model.set_datatype(aidge_core.dtype.float32)
model.set_backend("cpu")

model.save("PreFuse")
print("Fuse ConvScaling")

aidge_core.fuse_to_metaops(model, "Conv2D->Quantizer", "PaddedConvScalingRelu")
model.compile('cpu',aidge_core.dtype.float32, dims=[input_dims])

model.save("PostFuse")


scheduler = aidge_core.SequentialScheduler(model)
scheduler.generate_scheduling()

from aidge_export_arm_cortexm.export_registry import ExportLibAidgeARM, ExportLibCMSISNN

for node in model.get_nodes():

    aidge_core.Log.debug(f"Setting backend {ExportLibCMSISNN._name} to {node.name()}[{node.type()}].")
    node.get_operator().set_backend(ExportLibCMSISNN._name)


aidge_core.adapt_to_backend(model)

model.save("adapt_to_backend")
aidge_core.constant_folding(model)
aidge_core.constant_folding(model)
model.save("constant_folding")

# aidge_core.remove_constantOfShape(model)

aidge_core.expand_metaops(model)
aidge_core.expand_metaops(model)

model.save("exploded")
print("print")