fix: gracefully handle custom stake returning string

closes freqtrade#12421

Author: xmatthias

freqtrade/wallets.py

@@ -382,7 +382,7 @@ def validate_stake_amount(
         max_stake_amount: float,
         trade_amount: float | None,
     ):
-        if not stake_amount:
+        if not stake_amount or isinstance(stake_amount, str) or stake_amount <= 0:
             self._local_log(
                 f"Stake amount is {stake_amount}, ignoring possible trade for {pair}.",
                 level="debug",