Fix details.liquidHours() crash if no dates

Apparently sometimes the IBKR API doesn't populate all the date fields,
so attempting to parse empty timezone names crashes the date range
parsing logic.

Easy fix: if no dates available or if timezone isn't populated, just
return nothing (instead of crashing) because we can't do anything.

Fixes #46

Author: mattsta

ib_async/contract.py

@@ -582,6 +582,22 @@ def liquidSessions(self) -> List[TradingSession]:
         return self._parseSessions(self.liquidHours)
 
     def _parseSessions(self, s: str) -> List[TradingSession]:
+        """Parse the IBKR session date range text format into native Python objects.
+
+        Note: The IBKR date range format looks like:
+            timeZoneId='US/Eastern',
+            tradingHours='20240721:CLOSED;20240722:0400-20240722:2000;20240723:0400-20240723:'
+                '2000;20240724:0400-20240724:2000;20240725:0400-20240725:2000;'
+                '20240726:0400-20240726:2000',
+            liquidHours='20240721:CLOSED;20240722:0930-20240722:1600;20240723:0930-20240723:'
+                '1600;20240724:0930-20240724:1600;20240725:0930-20240725:1600;'
+                '20240726:0930-20240726:1600',
+        """
+
+        # if the time values don't exist, we can't parse anything, so return nothing.
+        if not (s or self.timeZoneId):
+            return []
+
         tz = util.ZoneInfo(self.timeZoneId)
         sessions = []
         for sess in s.split(";"):