fix(calling): refactor code to avoid getTime from a null variable

[lgmarchi]

What this PR does 📖

This pull request includes a minor change to the CallScreen.svelte file to improve the handling of the timeCallStarted variable. The change ensures that the variable is checked for null before being used.

• src/lib/components/calling/CallScreen.svelte: Modified the code to assign $timeCallStarted to a local variable timeCallStarted and added a null check before using it.

Which issue(s) this PR fixes 🔨

• Resolve #

Special notes for reviewers 🗒️

Additional comments 🎤

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• build-android.zip

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