fix: DH-16595: Fix null partition filter

[vbabich]

Closes #1867

Tested in Enterprise using steps from DH-16595 - created a partitioned table and set ACL to nullFilterGenerator() to get the null value in the partition column. Creating a partitioned table with a null in the partition column doesn't seem possible with db.addTablePartition in Enterprise.

t=db.t("LearnDeephaven", "StockTrades").where()
db.addPartitionedTableDefinition("<namespace>", "partTable", "Date", t.getDefinition())
test = db.t("<namespace>" , "partTable")

// Set Column ACL on `partTable` to `new NullFilterGenerator()` in swing

test = db.t("<namespace>" , "partTable")

Core+ Groovy console:

t = emptyTable(1).update("X=(String)null","Y=ii").partitionBy("X")

[mofojed]

Minor display string change, can you also include how this was tested in the PR description?

[vbabich]

Changed null to (null), added testing steps to the description.

[vbabich]

I tested this in a Core+ worker and it looks like the api returns undefined for a null partition key value. Will need to investigate a bit more.

[vbabich]

Partitioned table with a null key in DHE:

DHC shows an extra empty option element until the core PR is merged.

[vbabich]

Added nullish coalescing to account for undefined returned by the JSAPI for null table values

[mofojed]

Minor suggestion to include the core ticket number.