Support IS NULL and IS NOT NULL

[leiyuou]

This PR adds the support of parsing IS NULL and IS NOT NULL cypher query
Add unit tests

[ChunxuTang]

lgtm. Thanks for the work.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.61538% with 8 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@cc936e6). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
rust/lance-graph/src/parser.rs	91.66%	4 Missing ⚠️
rust/lance-graph/src/semantic.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #40   +/-   ##
=======================================
  Coverage        ?   81.93%           
=======================================
  Files           ?       23           
  Lines           ?     8310           
  Branches        ?        0           
=======================================
  Hits            ?     6809           
  Misses          ?     1501           
  Partials        ?        0           

Flag	Coverage Δ
rust-unittests	81.93% <84.61%> (?)

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