fix(source/nats): add missing default attribute for vectors

[benjamin-awd]

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What's changed and what's your intention?

Currently, the backoff and filter_subjects fields are required due to a missing default attribute, causing source creation to fail when they are not defined.

However, these fields should are supposed to be optional and should allow the creation of the source to proceed, even if they are not defined.

This PR adds the missing default attribute, which fixes the above issue.

Related to #17615

Checklist

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[xiangjinwu]

Does this PR imply the following problems:

• Without the serde(default) the required: false here is actually misleading
• Other attributes without serde(default) have the same problem

[benjamin-awd]

Based on our testing of this, the other attributes are unaffected. It seems that only the vector types are affected - my guess is that this is somehow needed due to the custom deserializer.

I drew reference from the Iceberg sink that uses a similar pattern

risingwave/src/connector/src/sink/iceberg/mod.rs

Lines 129 to 141 in 63b4d47

	#[serde(
	rename = "s3.path.style.access",
	default,
	deserialize_with = "deserialize_bool_from_string"
	)]
	pub path_style_access: bool,
	#[serde(
	rename = "primary_key",
	default,
	deserialize_with = "deserialize_optional_string_seq_from_string"
	)]
	pub primary_key: Option<Vec<String>>,

[xiangjinwu]

Thanks! Yes, it seems only required when using deserialize_with. We will see if we can fix the required: false in yaml.

[benjamin-awd]

Anyway just to confirm, these attributes are optional so I think it's fine to keep required: false :)

Thanks for taking a look!

[xiangjinwu]

Yes these 2 are now truly optional with default. I meant fixing the yaml generator when default is missing but deserialize_with is present.