Add type check for post password

[devansh016]

Add type check for post password field it should be a string.
Trac ticket: https://core.trac.wordpress.org/ticket/61837

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props devansh2002.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

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[desrosj]

@devansh016 While compiling a list of contributors that helped with the upcoming WordPress 6.7 release using the project's "Prop" script, your account came up as not having linked their WordPress.org and GitHub accounts.

In the WordPress project, all credit given out must be attached to a WordPress.org account. After creating a W.org account (or logging in to your preexisting one), you can link your GitHub account following these steps in the Core Handbook. If you could kindly link your .org account and just share your .org username, we can ensure you receive proper attribution! cc/@jeffpaul

[desrosj]

Ah, actually found your .org! https://profiles.wordpress.org/devansh2002/ Thanks for your contributions!