fix namespae def in da

Author: calad0i

hls4ml/backends/vivado/passes/distributed_arithmetic.py

@@ -218,7 +218,7 @@ def transform(self, model: 'ModelGraph', node: Layer):
 
         # override function_cpp
         io_type = node.model.config.get_config_value("IOType")
-        namespace = node.model.config.get_writer_config().get('Namespace', 'nnet')
+        namespace = node.model.config.get_writer_config().get('Namespace', None) or 'nnet'
         if io_type == 'io_parallel':
             fn_name = f'dense_da_{node.index}<{inp_t}, {out_t}>'
             function_cpp = f'{namespace}::{fn_name}({inp_name}, {out_name});'