[TEST][FEATURE] Support IS NOT NULL in PPL where clause

[qianheng-aws]

Is your feature request related to a problem?

Currently PPL does not fully support IS NOT NULL conditions in where clauses, making it difficult to filter out null values in queries.

What solution would you like?

Add support for IS NOT NULL condition in PPL where clause so users can filter documents where a specific field is not null.

Example

source=my_index | where field IS NOT NULL

What alternatives have you considered?

Using isnotnull(field) function as a workaround, but the SQL-standard syntax would be more intuitive.