Fix explain() failing on UnaryFunction with multiple children

python/cudf_polars/cudf_polars/streaming/explain.py

@@ -488,8 +488,11 @@ def _predicate_to_str(expr: Expr) -> str:
             sym = _BINOP_SYMBOLS.get(op, op.name)
             return f"({_predicate_to_str(left)} {sym} {_predicate_to_str(right)})"
         case UnaryFunction(name=name):
-            (child,) = expr.children
-            return f"{name}({_predicate_to_str(child)})"
+            # Unlike the other cases here, UnaryFunction doesn't have a fixed
+            # number of children. E.g. `pl.col("x").fill_null(0)` has two:
+            # the column expression ("x") and the fill value literal (0).
+            args = ", ".join(_predicate_to_str(child) for child in expr.children)
+            return f"{name}({args})"
         case Ternary():
             when, then, otherwise = expr.children
             return f"when({_predicate_to_str(when)}).then({_predicate_to_str(then)}).otherwise({_predicate_to_str(otherwise)})"