signer API: Fix unrecognized_fields

* do not check falsyness of argument containers: the results are
  unexpected if the argument container is empty
* Do not use Mapping annotation for unrecognized_fields. The original
  idea was that Metadata API is not changing them: this is true but we
  don't want to prevent users from modifying the fields...

securesystemslib/signer/_key.py

@@ -52,7 +52,11 @@ def __init__(
         self.keytype = keytype
         self.scheme = scheme
         self.keyval = keyval
-        self.unrecognized_fields = unrecognized_fields or {}
+
+        if unrecognized_fields is None:
+            unrecognized_fields = {}
+
+        self.unrecognized_fields = unrecognized_fields
 
     def __eq__(self, other: Any) -> bool:
         if not isinstance(other, Key):

securesystemslib/signer/_signature.py

@@ -1,7 +1,7 @@
 """Signature container class"""
 
 import logging
-from typing import Any, Dict, Mapping, Optional
+from typing import Any, Dict, Optional
 
 logger = logging.getLogger(__name__)
 
@@ -27,11 +27,15 @@ def __init__(
         self,
         keyid: str,
         sig: str,
-        unrecognized_fields: Optional[Mapping[str, Any]] = None,
+        unrecognized_fields: Optional[Dict[str, Any]] = None,
     ):
         self.keyid = keyid
         self.signature = sig
-        self.unrecognized_fields: Mapping[str, Any] = unrecognized_fields or {}
+
+        if unrecognized_fields is None:
+            unrecognized_fields = {}
+
+        self.unrecognized_fields = unrecognized_fields
 
     def __eq__(self, other: Any) -> bool:
         if not isinstance(other, Signature):