[Variant] If `ObjectBuilder::finalize` is not called, the resulting Variant object is malformed.

[alamb]

Describe the bug

If ObjectBuilder::finalize is not called, the resulting Variant object is malformed.

To Reproduce

Here is an example that would work with finalize on drop, but not with this PR as is

#[test]
    fn test_object_list_no_finish() {
        let mut builder = VariantBuilder::new();

        let mut list_builder = builder.new_list();

        {
            let mut object_builder = list_builder.new_object();
            object_builder.insert("id", 1);
            object_builder.finish().unwrap();
        }

        {
            let mut object_builder = list_builder.new_object();
            object_builder.insert("id", 2);
            // NOTE object_builder.finish() is not called here
        }

        list_builder.finish();

        let (metadata, value) = builder.finish();

        let variant = Variant::try_new(&metadata, &value).unwrap();
        let list = variant.as_list().unwrap();
}

If object_builder.finish() is not called, then this line fails:

        let list = variant.as_list().unwrap();

The error is like this (which is pretty unintuitive to me):

called `Result::unwrap()` on an `Err` value: InvalidArgumentError("Received empty bytes")
thread 'builder::tests::test_object_list_no_finish' panicked at parquet-variant/src/builder.rs:1154:59:
called `Result::unwrap()` on an `Err` value: InvalidArgumentError("Received empty bytes")
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/std/src/panicking.rs:697:5
   1: core::panicking::panic_fmt
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/panicking.rs:75:14
   2: core::result::unwrap_failed
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/result.rs:1732:5
   3: core::result::Result<T,E>::unwrap
             at /Users/andrewlamb/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/result.rs:1137:23
   4: parquet_variant::builder::tests::test_object_list_no_finish
             at ./src/builder.rs:1154:23
   5: parquet_variant::builder::tests::test_object_list_no_finish::{{closure}}
             at ./src/builder.rs:1133:36
   6: core::ops::function::FnOnce::call_once
             at /Users/andrewlamb/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
   7: core::ops::function::FnOnce::call_once
             at /rustc/6b00bc3880198600130e1cf62b8f8a93494488cc/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

**Expected behavior**
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**Additional context**
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[alamb]

label_issue.py automatically added labels {'parquet'} from #7843