fix: remove unreasonable nullable check for data types in hash_joiner during merge operation

[zhangyue19921010]

During the invocation of add columns at

lance/rust/lance/src/dataset/fragment.rs

Line 1589 in 8c18429

pub(crate) async fn merge(mut self, join_column: &str, joiner: &HashJoiner) -> Result<Self> {

, if the left table contains 100 rows of data and the right table contains 10 rows of data (with the remaining 90 rows being null), the validation at

lance/rust/lance/src/dataset/hash_joiner.rs

Line 192 in 8c18429

if array.null_count() > 0 && !lance_supports_nulls(array.data_type()) {

will fail.

This is unreasonable because the validation here is a legacy validation that is no longer applicable to V2_0 and above.

ValueError: Invalid user input: Found rows on LHS that do not match any rows on RHS. 
Lance would need to write nulls on the RHS, but Lance does not yet support nulls for type Float32.,
/Users/xxx/xxx/lance/rust/lance/src/dataset/hash_joiner.rs:197:36

[github-actions]

ACTION NEEDED
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[codecov]

Codecov Report

❌ Patch coverage is 51.11111% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/hash_joiner.rs	50.00%	18 Missing and 3 partials ⚠️
rust/lance/src/dataset/fragment.rs	66.66%	0 Missing and 1 partial ⚠️

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