Fix incorrect check in cache_unsigned_tx_pieces

[bigspider]

self.tx is not None is always true because the constructor sets self.tx = CTransaction() when None is passed, which causes a call to setup_from_tx also for PSBT version 2, contrarily to the function's stated intent in the documentation.

[brunoerg]

ACK 13715e8

Can be tested with this (vibe-coded) test:

diff --git a/test/test_psbt.py b/test/test_psbt.py
index 3a585b0..2c0c009 100755
--- a/test/test_psbt.py
+++ b/test/test_psbt.py
@@ -1,6 +1,6 @@
 #! /usr/bin/env python3

-from hwilib.psbt import PSBT
+from hwilib.psbt import PSBT, PartiallySignedInput, PartiallySignedOutput
 from hwilib.errors import PSBTSerializationError
 import json
 import os
@@ -28,5 +28,40 @@ class TestPSBT(unittest.TestCase):
                 serd = psbt.serialize()
                 self.assertEqual(valid, serd)

+    def test_v2_roundtrip_preserves_global_fields(self):
+        # Regression test: deserializing a PSBTv2 must not clobber the global
+        # tx_version and fallback_locktime fields. cache_unsigned_tx_pieces()
+        # used to call setup_from_tx() unconditionally, which for a v2 PSBT
+        # (whose self.tx is a default-constructed null CTransaction) overwrote
+        # tx_version with 1 and fallback_locktime with 0.
+        psbt = PSBT()
+        psbt.version = 2
+        psbt.explicit_version = True
+        psbt.tx_version = 2
+        psbt.fallback_locktime = 500000
+
+        psin = PartiallySignedInput(2)
+        psin.prev_txid = bytes.fromhex("11" * 32)
+        psin.prev_out = 0
+        psin.sequence = 0xFFFFFFFF
+        psbt.inputs.append(psin)
+
+        psout = PartiallySignedOutput(2)
+        psout.amount = 12345
+        psout.script = b"\x00\x14" + b"\x22" * 20
+        psbt.outputs.append(psout)
+
+        serd = psbt.serialize()
+
+        deser = PSBT()
+        deser.deserialize(serd)
+
+        self.assertEqual(deser.version, 2)
+        self.assertEqual(deser.tx_version, 2)
+        self.assertEqual(deser.fallback_locktime, 500000)
+        self.assertEqual(deser.inputs[0].prev_out, 0)
+        self.assertEqual(deser.outputs[0].amount, 12345)
+        self.assertEqual(serd, deser.serialize())
+
 if __name__ == "__main__":
     unittest.main()