dtoc: Tidy up implementation of AddStringList()

Refactor this to avoid a loop. Also add a test for an empty string.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
diff --git a/tools/dtoc/fdt_util.py b/tools/dtoc/fdt_util.py
index 5755062..d7c38ad 100644
--- a/tools/dtoc/fdt_util.py
+++ b/tools/dtoc/fdt_util.py
@@ -158,6 +158,8 @@
     if not prop:
         return default
     value = prop.value
+    if not prop.bytes:
+        return ''
     if isinstance(value, list):
         raise ValueError("Node '%s' property '%s' has list value: expecting "
                          "a single string" % (node.name, propname))
@@ -179,6 +181,8 @@
     if not prop:
         return default
     value = prop.value
+    if not prop.bytes:
+        return []
     if not isinstance(value, list):
         strval = GetString(node, propname)
         return [strval]