fix: Cypress.dom.isJquery() returns only boolean results

[sainthkh]

• Closes Cypress.dom.isJquery is inconsistent in return #14278

User facing changelog

isJquery returns only boolean results.

Additional details

• Why was this change necessary? => isJquery sometimes returned undefined.
• What is affected by this change? => N/A
• Any implementation details to explain? => Simply added !!.

How has the user experience changed?

Users will not get undefined results.

PR Tasks

• Have tests been added/updated?

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[jennifer-shehane]

Thanks! This fixes the original issue.