mypy: Rewrite sign_is_valid in zthumbor helpers.py for None secret_key.

mypy.ini

@@ -89,9 +89,6 @@ strict_optional = False
 [mypy-zilencer.management.commands.add_new_realm]  #22: error: List item 0 has incompatible type "Optional[Stream]"; expected "Stream"
 strict_optional = False
 
-[mypy-zthumbor.loaders.helpers]  #83: error: Argument 2 to "get_sign_hash" has incompatible type "Optional[str]"; expected "str"
-strict_optional = False
-
 # Re-architecting required?
 
 [mypy-zerver.lib.queue]  # Delayed setup of SimpleQueueClient.channel (Optional)

zthumbor/loaders/helpers.py

@@ -80,6 +80,7 @@ def sign_is_valid(url, context):
         force_text(size),
         force_text(source_type),
     ])
-    if sign == get_sign_hash(raw, get_secret('thumbor_key')):
-        return True
-    return False
+    secret_key = get_secret('thumbor_key')
+    if secret_key is None or sign != get_sign_hash(raw, secret_key):
+        return False
+    return True