imgtool: Fix 'custom_tlvs' argument handling

When Image.create() is called without the 'custom_tlvs' argument, it
gets its default value (None). It must be checked before performing
any operations on/with it.

Change-Id: I8e0755265f35f9eeb796fe078a6ad8c8d9f2b8da
Signed-off-by: David Vincze <david.vincze@linaro.org>

scripts/imgtool/image.py

@@ -1,5 +1,5 @@
 # Copyright 2018 Nordic Semiconductor ASA
-# Copyright 2017 Linaro Limited
+# Copyright 2017-2020 Linaro Limited
 # Copyright 2019-2020 Arm Limited
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -327,7 +327,7 @@ class Image():
             dependencies_num = len(dependencies[DEP_IMAGES_KEY])
             protected_tlv_size += (dependencies_num * 16)
 
-        if custom_tlvs:
+        if custom_tlvs is not None:
             for value in custom_tlvs.values():
                 protected_tlv_size += TLV_SIZE + len(value)
 
@@ -375,6 +375,7 @@ class Image():
                                     )
                     prot_tlv.add('DEPENDENCY', payload)
 
+            if custom_tlvs is not None:
                 for tag, value in custom_tlvs.items():
                     prot_tlv.add(tag, value)