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 87.44%. Comparing base (1962a64) to head (33faee1).

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   87.44%   87.44%           
=======================================
  Files         128      128           
  Lines       11368    11384   +16     
  Branches     1545     1549    +4     
=======================================
+ Hits         9941     9955   +14     
- Misses       1038     1039    +1     
- Partials      389      390    +1     

Flag	Coverage Δ
datachain	87.39% <93.54%> (+<0.01%)	⬆️

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