Custom expression: new function ISNULL() and ISEMPTY()

[ariya]

These new functions are to support MBQL's is-null and is-empty. This is to address issue #12225.

Steps to verify

1. Ask a question, Custom question
2. Sample Dataset, Reviews table
3. Filter, Add filters to narrow your answer, Custom Expression
4. Type NOT isempty([Reviewer] ), click Done
5. Click on the + button, another Custom Expression
6. Type NOT isnull([Rating]), click Done
7. Visualize

Before this PR

It was not possible, since custom expression's IsNull and IsEmpty functions did not exist.

After this PR

Additional Note

The simple filter "Is empty" can be also (properly) converted to a custom expression:

1. Ask a question, Simple question, Reviews
2. Filter, Reviewer, Is empty and then click Add filter
3. Click on the new "Reviewer is empty" filter
4. Click on the left chevron icon, Custom Expression

The same also works for Discount:

Note that is-null vs is-empty depends on the field type, see PR #12947 for more details.

Related issue: negative filter doesn't get converted yet, see #14880 for more details.

[flamber]

For historians, might wanna look in #5299 - I think it's great to have specific functions in CE, whereas the regular filter options is fine having "Is empty", which is (column IS NULL OR column = '')

[flamber]

Works great! 🎉 And the code is just like the other functions, now the backend is in place.
I can even do case(isnull([Field]), "NULL", NOT isnull([Field]), "NOT NULL") for a CC