feat: add (isNull, checkDomain) util function

[klmhyeonwoo]

closes: #16
closes: #14

[Copilot]

Pull Request Overview

This PR adds a new isNull utility function that provides a type guard for checking if a value is strictly null. The implementation includes comprehensive test coverage and documentation updates.

• Implements a type-safe null checking utility with TypeScript type guard
• Adds comprehensive test cases covering various falsy values
• Updates package exports and documentation with usage examples

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
package/commonUtil/isNull/index.ts	Core implementation of the isNull type guard function
package/commonUtil/isNull/index.test.ts	Test suite covering null and non-null value scenarios
package/commonUtil/index.ts	Exports the new isNull function from the package
README.md	Updates documentation with usage examples and API reference

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[klmhyeonwoo]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	47.15%	83 / 176
🔵	Statements	47.15%	83 / 176
🔵	Functions	59.09%	13 / 22
🔵	Branches	81.13%	43 / 53

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
package/commonUtil/index.ts	0%	0%	0%	0%	1-2
package/commonUtil/isNull/index.ts	100%	100%	100%	100%
package/validationUtil/index.ts	0%	0%	0%	0%	1-3
package/validationUtil/checkDomain/index.ts	100%	100%	100%	100%

Generated in workflow #31 for commit c7098dc by the Vitest Coverage Report Action

[prgmr99]

고생하셨습니다~!!