[test](geo) drop redundant assert_cast on already-typed null_map column

### What problem does this PR solve?

Issue Number: close #N/A

Problem Summary:

BE-UT has been failing master-wide since both PR #63491 (which strongly
typed `ColumnNullable::get_null_map_column[_ptr]()` to `ColumnUInt8`) and
PR #63049 (which added `functions_geo_test.cpp`) landed today. The new
test calls

```cpp
assert_cast<ColumnUInt8*>(nullable_input->get_null_map_column_ptr().get())
        ->insert_value(0);
```

but the inner expression is already `ColumnUInt8*`, so the cast triggers
the same-type static_assert added by PR #63133 to `src/core/assert_cast.h`:

```
static assertion failed due to requirement
'!std::is_same_v<doris::ColumnVector<doris::TYPE_BOOLEAN> *,
                 doris::ColumnVector<doris::TYPE_BOOLEAN> *>':
    assert_cast is redundant for the same type after removing cv/ref qualifiers
```

That kills `doris_be_test` compilation on every PR that runs BE-UT.

Use the strongly typed `get_null_map_column()` (which returns
`ColumnUInt8&`) directly so the cast is no longer needed.

### Release note

None (test-only change, restores BE-UT compilation on master).

### Check List (For Author)

- Test:
    - Compile-check on local ASAN tree: the affected translation unit
      now builds clean (`ninja test/CMakeFiles/doris_be_test.dir/exprs/function/geo/functions_geo_test.cpp.o`).
- Behavior changed: No
- Does this need documentation: No

Author: airborne12

be/test/exprs/function/geo/functions_geo_test.cpp

@@ -372,7 +372,10 @@ TEST(VGeoFunctionsTest, function_geo_st_geometries_invalid) {
     // Insert non-null but invalid data
     auto* nullable_input = assert_cast<ColumnNullable*>(input_col.get());
     nullable_input->get_nested_column_ptr()->insert_data(invalid_buf.data(), invalid_buf.size());
-    assert_cast<ColumnUInt8*>(nullable_input->get_null_map_column_ptr().get())->insert_value(0);
+    // After #63491, ColumnNullable::get_null_map_column() already returns
+    // ColumnUInt8&, so the previous assert_cast<ColumnUInt8*>(...) is now
+    // rejected by the same-type static_assert in src/core/assert_cast.h.
+    nullable_input->get_null_map_column().insert_value(0);
     block.insert({std::move(input_col), input_type, "shape"});
 
     FunctionBasePtr func = SimpleFunctionFactory::instance().get_function(