fix isin w nulls

Author: TrevorBergeron

bigframes/core/compile/sqlglot/expressions/comparison_ops.py

@@ -37,6 +37,7 @@ def _(expr: TypedExpr, op: ops.IsInOp) -> sge.Expression:
     must_upcast_bools = dtypes.is_numeric(expr.dtype, include_bool=False) or any(
         dtypes.is_numeric(dtypes.bigframes_type(type(value)), include_bool=False)
         for value in op.values
+        if not _is_null(value)
     )
     for value in op.values:
         if _is_null(value):

bigframes/core/rewrite/identifiers.py

@@ -72,7 +72,7 @@ def remap_variables(
         visible to the parent node.
     """
     # step 1: defined remappings for each individual unique node
-    # step 2: bottom up traversal to apply remappings
+    # step 2: top down traversal to apply remappings (mappings are value-based, so bottom-up doesn't work)
 
     id_def_remaps: dict[
         nodes.BigFrameNode, dict[identifiers.ColumnId, identifiers.ColumnId]