fix[scalar_fns]: add correct pre-condition checks for all custom pushdown (#6977)

Check the fill null value

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>

Author: joseph-isaacs

vortex-array/src/scalar_fn/fns/fill_null/kernel.rs

@@ -3,6 +3,7 @@
 
 use vortex_error::VortexExpect;
 use vortex_error::VortexResult;
+use vortex_error::vortex_ensure;
 
 use crate::ArrayRef;
 use crate::ExecutionCtx;
@@ -57,6 +58,11 @@ pub(super) fn precondition(
     array: &ArrayRef,
     fill_value: &Scalar,
 ) -> VortexResult<Option<ArrayRef>> {
+    vortex_ensure!(
+        !fill_value.is_null(),
+        "fill_null requires a non-null fill value"
+    );
+
     // If the array has no nulls, fill_null is a no-op (just cast for nullability).
     if !array.dtype().is_nullable() || array.all_valid()? {
         return array.to_array().cast(fill_value.dtype().clone()).map(Some);

vortex-array/src/scalar_fn/fns/fill_null/mod.rs

@@ -105,6 +105,11 @@ impl ScalarFnVTable for FillNull {
             .as_constant()
             .ok_or_else(|| vortex_err!("fill_null fill_value must be a constant/scalar"))?;
 
+        vortex_ensure!(
+            !fill_scalar.is_null(),
+            "fill_null requires a non-null fill value"
+        );
+
         let Some(columnar) = input.as_opt::<AnyColumnar>() else {
             return input.execute::<ArrayRef>(ctx)?.fill_null(fill_scalar);
         };