Add a guard to prevent errors in the onCLS() function

[philipwalton]

This PR fixes an error where the CLS session window logic tries to access startTime of an undefined entry.

It's still a mystery how this can possibly happen, but given that at least three people have reported it over the years. It's probably worth adding a guard for:

Addresses the following issues:

• TypeError: Cannot read properties of undefined (reading 'startTime') #274
• chore: optimize sessionValue check in onCLS.ts #521
• Uncaught TypeError: Cannot read properties of undefined (reading 'startTime') in getCLS logic getsentry/sentry-javascript#6083

(Note: the base branch here is temporary to make the diff easier to see. I'll updated it after the other PRs are merged into the v5 branch.)