Add Null Type Coercions for Placeholders (#20543)

## Which issue does this PR close?

At least this one: #19471 but
might be others

## Rationale for this change

This fixes a problem we have where placeholder types are `Null`, and we
need to add explicit type casts in the query.

I.e, you can't use placeholders for `date_bin` functions:

```sql
select date_bin($resolution, timestamp) 
```

This works though (currently):

```sql
select date_bin($resolution::interval, timestamp) 
```

## What changes are included in this PR?

Adds some extra type coercions

## Are these changes tested?

Yes, new tests added

## Are there any user-facing changes?

Not really?

Author: cetra3

datafusion/expr/src/type_coercion/functions.rs

@@ -928,6 +928,13 @@ fn coerced_from<'a>(
         (Timestamp(_, Some(_)), Null | Timestamp(_, _) | Date32 | Utf8 | LargeUtf8) => {
             Some(type_into.clone())
         }
+        // Null can be coerced to any target type, provided the cast is valid.
+        // This mirrors null_coercion() in binary comparison coercion
+        // (expr-common/src/type_coercion/binary.rs) and is the symmetric
+        // counterpart of the (Null, _) arm above. Without this, untyped
+        // placeholders ($1, $foo) inside function calls fail signature matching
+        // because their Null type doesn't match any Exact(...) variant.
+        (_, Null) if can_cast_types(type_from, type_into) => Some(type_into.clone()),
         _ => None,
     }
 }
@@ -937,7 +944,7 @@ mod tests {
     use crate::Volatility;
 
     use super::*;
-    use arrow::datatypes::Field;
+    use arrow::datatypes::{Field, IntervalUnit};
     use datafusion_common::{
         assert_contains,
         types::{logical_binary, logical_int64},
@@ -956,6 +963,36 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_coerced_from_null() {
+        // Null should coerce to Interval (the motivating case)
+        assert_eq!(
+            coerced_from(
+                &DataType::Interval(IntervalUnit::MonthDayNano),
+                &DataType::Null
+            ),
+            Some(DataType::Interval(IntervalUnit::MonthDayNano))
+        );
+
+        // Null should coerce to Date32
+        assert_eq!(
+            coerced_from(&DataType::Date32, &DataType::Null),
+            Some(DataType::Date32)
+        );
+
+        // Null should coerce to Timestamp with timezone
+        assert_eq!(
+            coerced_from(
+                &DataType::Timestamp(TimeUnit::Microsecond, Some("+00".into())),
+                &DataType::Null
+            ),
+            Some(DataType::Timestamp(
+                TimeUnit::Microsecond,
+                Some("+00".into())
+            ))
+        );
+    }
+
     #[test]
     fn test_maybe_data_types() {
         // this vec contains: arg1, arg2, expected result

datafusion/sqllogictest/test_files/datetime/timestamps.slt

@@ -5385,3 +5385,9 @@ SELECT to_timestamp(arrow_cast(100.5, 'Float16'), name) FROM test_to_timestamp_s
 
 statement ok
 drop table test_to_timestamp_scalar
+
+# date_bin with NULL interval should return NULL, not a planning error
+query P
+SELECT date_bin(NULL, TIMESTAMP '2023-01-01 12:30:00', TIMESTAMP '2023-01-01 12:00:00')
+----
+NULL

datafusion/sqllogictest/test_files/nvl.slt

@@ -114,7 +114,7 @@ SELECT NVL(1, 3);
 ----
 1
 
-query I
+query B
 SELECT NVL(NULL, NULL);
 ----
 NULL