Add REMOVE_NULLS function for arrays

[sviscaino]

Currently, to remove nulls from an array one needs to write FILTER(arr, x -> x IS NOT NULL).
This creates a new REMOVE_NULLS function to strip nulls from an array.
Test plan -

mvn -Dtest="TestArrayOperators" test

== RELEASE NOTES ==

General Changes
* Add :func:`remove_nulls` function to remove null elements from an array

[kaikalur]

This might cause a lot of issues as we are using equality (=) operator for most array operations. So NULL should not be allowed here like this.

I suggest adding a new operator REMOVE_NULLS that does it.

[sviscaino]

This might cause a lot of issues as we are using equality (=) operator for most array operations. So NULL should not be allowed here like this.

I suggest adding a new operator REMOVE_NULLS that does it.

That makes sense! I have updated the PR code and description accordingly.

[kaikalur]

Also, please add documentation for the new function:

https://github.com/prestodb/presto/blob/master/presto-docs/src/main/sphinx/functions/array.rst

Just like for the others.

[sviscaino]

Also, please add documentation for the new function:

https://github.com/prestodb/presto/blob/master/presto-docs/src/main/sphinx/functions/array.rst

Just like for the others.

Yes I have added documentation in array.rst

[kaikalur]

LGTM

[kaikalur]

Please fix the build - checkstyle violations.

[sviscaino]

Please fix the build - checkstyle violations.

Oops, done

[highker]

LGTM; @kaikalur feel free to merge this PR