Remove empty hash dict check from MetaFile

The check for an empty hash dictionary is now part
of the hash validation function.

Signed-off-by: Teodora Sechkova <[email protected]>

Author: sechkova

tuf/api/metadata.py

@@ -793,8 +793,7 @@ def verify_length_and_hashes(self, data: Union[bytes, BinaryIO]):
         if self.length is not None:
             self._verify_length(data, self.length)
 
-        # Skip the check in case of an empty dictionary too
-        if self.hashes:
+        if self.hashes is not None:
             self._verify_hashes(data, self.hashes)