feat: add isnullish builtin function

[Peefy]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y

Closes #1600

feat: add isnullish builtin function

[zong-zhe]

LGTM

[coveralls]

Pull Request Test Coverage Report for Build 10575292567

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 1 of 14 (7.14%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 70.915%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
kclvm/runtime/src/stdlib/builtin_api.rs	0	13	0.0%

Totals
Change from base Build 10571823119:	-0.01%
Covered Lines:	51575
Relevant Lines:	72728

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💛 - Coveralls