date NaN logic (#1737)

Co-authored-by: RamilCDISC <113539111+RamilCDISC@users.noreply.github.com>

Author: SFJohnson24

cdisc_rules_engine/check_operators/helpers.py

@@ -56,7 +56,7 @@ def default_value(self):
 
 
 def is_valid_date(date_string: str) -> bool:
-    if date_string is None:
+    if date_string is None or not isinstance(date_string, str):
         return False
     try:
         isoparse(date_string)