tlv: Fix encoding of zero-valued TLVs

If a TLV was elementary (no nested IEs), and it had only a single
integer content whose value is 0, we erroneously encoded that as
zero-length TLV (len=0, no value part):

>>> rf = pySim.euicc.RefreshFlag(decoded=0);
>>> rf.to_bytes()
b''
>>> rf.to_tlv()
b'\x81\x00'

After this change it is correct:

>>> rf = pySim.euicc.RefreshFlag(decoded=0);
>>> rf.to_bytes()
b'\x00'
>>> rf.to_tlv()
b'\x81\x01\x00'

Change-Id: I5f4c0555cff7df9ccfc4a56da12766d1bf89122f

pySim/tlv.py

@@ -87,7 +87,7 @@ class Transcodable(abc.ABC):
     def to_bytes(self) -> bytes:
         """Convert from internal representation to binary bytes.  Store the binary result
         in the internal state and return it."""
-        if not self.decoded:
+        if self.decoded == None:
             do = b''
         elif self._construct:
             do = self._construct.build(self.decoded, total_len=None)