Added isnone() function

[ilongin]

isnone(...) function returns True if column or literal value is None / NULL, otherwise False

Note that this doesn't currently work for Clickhouse where we don't have nullable columns. We should decide if we want to introduce nullable columns and if yes, this will work there as well.

UPDATE:
In addition, this PR extends case() function and related codebase to accept nested functions in:

1. Case condition - value (both places)
2. Case else_ value

This will allow usage of functions inside other functions that are based on case like ifelse or isnone as well. We also will have ability to do nested cases etc.

[codecov]

Codecov Report

Attention: Patch coverage is 93.54839% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.42%. Comparing base (d648134) to head (e433f48).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/datachain/func/func.py	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #801      +/-   ##
==========================================
- Coverage   87.45%   87.42%   -0.03%     
==========================================
  Files         128      128              
  Lines       11408    11424      +16     
  Branches     1553     1557       +4     
==========================================
+ Hits         9977     9988      +11     
- Misses       1041     1044       +3     
- Partials      390      392       +2     

Flag	Coverage Δ
datachain	87.36% <93.54%> (-0.03%)	⬇️

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