Better handle if no valid signs are found

Author: theseal

src/pyff/utils.py

@@ -273,7 +273,9 @@ def check_signature(t: ElementTree, keys: list = [], only_one_signature: bool =
             except xmlsec.exceptions.XMLSigException:
                 continue
 
-        if refs:
+        if not refs:
+            raise MetadataException("No valid signature(s) found")
+        else:
             if only_one_signature and len(refs) != 1:
                 raise MetadataException("XML metadata contains %d signatures - exactly 1 is required" % len(refs))
             t = refs[0]  # prevent wrapping attacks