MLBEDSW-2915: Added None check for bias before checking restrictions

Signed-off-by: Jacob Bohlin <jacob.bohlin@arm.com>
Change-Id: I04618fd0d29075e7d3f8f27a320129603f045163
diff --git a/ethosu/vela/supported_operators.py b/ethosu/vela/supported_operators.py
index 591f080..7cff0ee 100644
--- a/ethosu/vela/supported_operators.py
+++ b/ethosu/vela/supported_operators.py
@@ -416,11 +416,11 @@
 
     def check_bias_restrictions(self, bias_tensor):
         # check data type
-        if bias_tensor.dtype not in (DataType.int32, DataType.int64):
+        if bias_tensor is not None and bias_tensor.dtype not in (DataType.int32, DataType.int64):
             return False
 
         # check if values fits in 40-bit
-        if bias_tensor.dtype == DataType.int64:
+        if bias_tensor is not None and bias_tensor.dtype == DataType.int64:
             for quant_value in bias_tensor.quant_values:
                 if not (-(1 << 39) <= quant_value < (1 << 39)):
                     return False