cal.com fix: Ensure previous slots do appear if the month search param is removed

Ensure correct fallback to current month when 'month' query param is missing

Updated the query parameter handling logic to properly format the 'date' param as 'YYYY-MM' in case month param is missing. This resolves an issue where previous slots were not appearing.

What does this PR do?

Fixes #18094 (GitHub issue number)

Mandatory Tasks (DO NOT REMOVE)

[ ] I have self-reviewed the code (A decent size PR without self-review might be rejected).
[ ] I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.
[ ] I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

Navigate to an Event Page: Open a public page for any event.
Select a Date: Choose a date from the middle or end of the month [Ensure there exists dates prior to the date you select for this scenario].
Modify the URL: Copy the page URL and manually remove the month query parameter.
Perform a Hard Refresh: Refresh the page while retaining the modified URL.

Expected Behavior

Before the Fix: Slots before the selected date were not selectable.
After the Fix: All slots, including those before the selected date, should now be selectable.