Fix % format comma separator and empty positions.json parse error

portfolio.py

@@ -49,7 +49,10 @@ class Portfolio:
         if not self._path.exists():
             return
         try:
-            data = json.loads(self._path.read_text())
+            content = self._path.read_text().strip()
+            if not content:
+                return
+            data = json.loads(content)
             self.positions = {pair: Position(**fields) for pair, fields in data.items()}
             log.info("Loaded %d position(s) from %s", len(self.positions), self._path)
         except Exception as exc: