branch-4.0 [opt](nereids) optimize estimation of num_nulls for outer join

[englefly]

What problem does this PR solve?

pick #57863
After performing an outer join, the columns on the inner side need to be supplemented with nulls. Taking a left outer join as an example, the previous estimation method was [left table row count] - [left semi join row count].
However, due to the large estimation error of the left semi join, the error in supplementing null values was relatively large. Now, it has been changed to left_table_row_count - inner_join_row_count.

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

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• Behavior changed:

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